Method for manufacturing gallium nitride crystal and gallium nitride wafer

ABSTRACT

There is provided a method for fabricating a gallium nitride crystal with low dislocation density, high crystallinity, and resistance to cracking during polishing of sliced pieces by growing the gallium nitride crystal using a gallium nitride substrate including dislocation-concentrated regions or inverted-polarity regions as a seed crystal substrate. Growing a gallium nitride crystal  79  at a growth temperature higher than 1,100° C. and equal to or lower than 1,300° C. so as to bury dislocation-concentrated regions or inverted-polarity regions  17   a  reduces dislocations inherited from the dislocation-concentrated regions or inverted regions  17   a , thus preventing new dislocations from occurring over the dislocation-concentrated regions or inverted-polarity regions  17   a . This also increases the crystallinity of the gallium nitride crystal  79  and its resistance to cracking during the polishing.

TECHNICAL FIELD

The present invention relates to a method for fabricating a galliumnitride crystal, and also relates to a gallium nitride wafer.

BACKGROUND ART

Patent Document 1 describes a method for manufacturing a single-crystalgallium nitride crystal at a uniform epitaxial growth rate, and thiscrystal has good cleavage and low dislocation density. According to thismethod, after numerous isolated shielding portions are formed on asubstrate of a different crystal, gallium nitride is grown byvapor-phase deposition into a sufficiently thick gallium nitride crystalwith closed defect-concentrated regions H extending over the shieldingportions. Subsequently, a free-standing gallium nitride frame substratewith a thickness of 100 to 500 μm is prepared by removing the backsubstrate and polishing the gallium nitride crystal or by cutting thecrystal parallel to its surface. The closed defect-concentrated regionsare then removed from the gallium nitride frame substrate by dry etchingwith hydrogen chloride gas, thus obtaining a gallium nitride skeletalsubstrate including only single-crystal low-dislocation-densityaccompanying regions and a single-crystal low-dislocation-densityremaining region. A less-defect-concentrated gallium nitride crystalwithout closed defect-concentrated regions is manufactured by growinggallium nitride on the gallium nitride skeletal substrate by vapor-phasedeposition.

Patent Document 2 describes a method for manufacturing a single-crystalsubstrate. According to this method, a crystal including both a portionwith polarity A and a portion with polarity B is used. The portion withpolarity B is completely or partially removed by etching to form a base,and a crystal is grown thereon again so that a crystal with polarity Ais formed over the surface of the base or the entire single crystal withpolarity A is formed thereon. Alternatively, the portion with polarity Bis covered with a different material after being partially removed ornot being removed, and the same crystal is grown again so that thesurface is covered with a crystal with polarity A. The single-crystalsubstrate manufactured by this method has a surface of a single crystalwith polarity A which is suitable for formation of an electronic devicethereon.

Patent Document 3 describes a method for growing single-crystal galliumnitride and for manufacturing a single-crystal gallium nitridesubstrate. According to this method, a mask with a regularly stripedpattern is provided on a base substrate, and linear V-grooves (troughs)defined by facets are formed thereover. Gallium nitride is facet-grownwhile the V-grooves are maintained so that defect-concentrated regions Hare formed on the bottoms of the V-grooves (troughs) defined by thefacets. Consequently, dislocations are concentrated to form thedefect-concentrated regions H and therefore results in fewerdislocations in the surrounding regions, namely, low-dislocation-densitysingle-crystal regions Z and c-plane growth regions Y. Thedefect-concentrated regions H trap and do not release the dislocationsthat have traped because the regions H are closed. Facet growth, inwhich gallium nitride is grown while facets are formed and maintained,has a disadvantage of forming dislocations extending from the centers ofpits defined by facets; they form planar defects extending radially. Inaddition, a device cannot be provided thereon because the positionswhere pits are formed cannot be controlled. These can be alleviated bythe method described in Patent Document 3.

Patent Document 4 describes a method for growing single-crystal galliumnitride and for fabricating single-crystal gallium nitride. According tothis method, a regular seed pattern is provided on a base substrate, andgallium nitride is facet-grown thereon while pits defined by facets areformed and maintained so that closed defect-concentrated regions H areformed on the bottoms of the pits defined by the facets. As a result,dislocations are concentrated to form the defect-concentrated regions H,while the dislocation density of the surrounding regions, namely,single-crystal low-dislocation-density accompanying regions Z and asingle-crystal low-dislocation-density remaining region Y, is reduced.The closed defect-concentrated regions H trap and do not release thedislocations because the regions H are closed.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2004-59363-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2004-221480-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2003-183100-   Patent Document 4: Japanese Unexamined Patent Application    Publication No. 2003-165799

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

A gallium nitride crystal has attracted attention as a material forsemiconductor devices, such as short-wavelength semiconductor opticaldevices and power electronic devices, because it has a bandgap energy of3.4 eV (5.4×10⁻¹⁹ J) and high thermal conductivity. One importantquality of materials for semiconductor devices is to contain fewercrystal dislocations. To achieve this quality, the development oflow-dislocation-density gallium nitride crystals and their substrateshas been pushed forward.

In Patent Documents 3 and 4, for example, a gallium nitride crystal isgrown such that dislocations concentrate in parts of the crystal, thusfabricating a partially low-dislocation-density gallium nitride crystal.Substrates manufactured by the methods described in Patent Documents 3and 4 include dislocation-concentrated regions or inverted regions. InPatent Documents 1 and 2, on the other hand, a gallium nitride crystalis grown on a gallium nitride crystal wafer described in Patent Document3 or 4, used as a seed crystal substrate so as to embeddislocation-concentrated regions or inverted regions, in order toprevent these regions from affecting the gallium nitride crystal beinggrown. The gallium nitride crystal manufactured by this method has adislocation density of about 1×10⁶ cm⁻².

If, however, a gallium nitride crystal is grown on a seed crystalsubstrate composed of a crystal including dislocation-concentratedregions or inverted polarity regions, as described in Patent Documents 1and 2, new dislocations may occur from sites where thedislocation-concentrated regions or inverted polarity regions areembedded. The grown gallium nitride crystal also has variations incrystallinity due to variations in the crystallinity of thedislocation-concentrated regions or inverted polarity regions of theseed crystal substrate, and these variations are not negligible in somecases. Furthermore, when the grown gallium nitride crystal is sliced andpolished to prepare a gallium nitride substrate, it may be crackedduring the polishing. This may preclude reliable production of galliumnitride substrates.

An object of the present invention, which has been made in light of theabove circumstances, is to provide a method for fabricating a galliumnitride crystal with low dislocation density, high crystallinity, andresistance to cracking during polishing after slicing by growing thegallium nitride crystal on a seed crystal substrate includingdislocation-concentrated regions or inverted regions, and also toprovide a gallium nitride wafer.

Means for Solving the Problems

An aspect of the present invention is a method for fabricating a galliumnitride crystal. This method includes the steps of (a) preparing agallium nitride substrate including a plurality of first regions havinga dislocation density higher than a first dislocation density, a secondregion having a dislocation density lower than the first dislocationdensity, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)forming recesses in the first areas to prepare a seed crystal substrate;and (c) growing a gallium nitride crystal on the seed crystal substrateby vapor-phase deposition such that voids corresponding to the recessesare formed.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate by vapor-phase deposition such that the voidscorresponding to the recesses are formed while the gallium nitridecrystal is integrally grown on the second region. The gallium nitridecrystal is therefore grown not in the first areas but in the second areaon the seed crystal substrate. Accordingly, the gallium nitride crystalhas a lower dislocation density because gallium nitride crystalinheriting dislocations from the first regions is not formed.

An aspect of the present invention is a method for fabricating a galliumnitride crystal. This method includes the steps of (a) preparing agallium nitride substrate including a plurality of first regions havinga dislocation density higher than a first dislocation density, a secondregion having a dislocation density lower than the first dislocationdensity, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)forming recesses in the first areas to prepare a seed crystal substrate;and (c) growing a gallium nitride crystal on the seed crystal substrateby liquid-phase deposition such that voids corresponding to the recessesare formed.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate by liquid-phase deposition such that the voidscorresponding to the recesses are formed while the gallium nitridecrystal is integrally grown on the second region. The gallium nitridecrystal is therefore grown not in the first areas but in the second areaon the seed crystal substrate. Accordingly, the gallium nitride crystalhas a lower dislocation density because gallium nitride crystalinheriting dislocations from the first regions is not formed.

An aspect of the present invention is a method for fabricating a galliumnitride crystal. This method includes the steps of (a) preparing agallium nitride substrate including a plurality of first regions havinga dislocation density higher than a first dislocation density, a secondregion having a dislocation density lower than the first dislocationdensity, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)preparing a seed crystal substrate by forming recesses in the firstareas; and (c) growing a gallium nitride crystal on the seed crystalsubstrate by vapor-phase deposition at a growth temperature higher than1,100° C. and equal to or lower than 1,300° C.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate by vapor-phase deposition such that the voidscorresponding to the recesses are formed while the gallium nitridecrystal is integrally grown on the second region. The gallium nitridecrystal is therefore grown not in the first areas but in the second areaon the seed crystal substrate. Accordingly, the gallium nitride crystalhas a lower dislocation density because no gallium nitride crystalinheriting dislocations from the first regions is formed. In addition,growing the gallium nitride crystal on the seed crystal substrate at agrowth temperature higher than 1,100° C. reduces new dislocationsoccurring when the gallium nitride crystal grown in the second area isintegrally grown over the recesses and also reduces the full width athalf maximum of an XRD (004) rocking curve. Another advantage is thatthe grown gallium nitride crystal can be prevented from being crackedduring polishing after slicing, thus improving the yield after thepolishing. Although it is unclear how cracking is prevented during thepolishing, it is assumed that a growth temperature higher than 1,100° C.will reduce stress-concentrated sites in the gallium nitride crystal. Agrowth temperature exceeding 1,300° C., on the other hand, causesdamaging due to accelerated decomposition of the seed crystal substrateand a significant decrease in the growth rate of the gallium nitridecrystal. It is generally believed that raising the growth temperatureincreases the formation rate of a gallium nitride crystal and thereforeincreases its growth rate. It is assumed, however, that thedecomposition rate of the gallium nitride crystal that has been formedincreases more significantly than the formation rate of the galliumnitride crystal at extraordinarily high growth temperatures, thusdecreasing the growth rate depending upon the difference between theformation rate and the decomposition rate. Accordingly, extraordinarilyhigh growth temperatures are undesirable, and the growth temperature ispreferably equal to or lower than 1,300° C.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. In addition, the growth temperatureis preferably equal to or lower than 1,250° C. According to thisinvention, growing the gallium nitride crystal on the seed crystalsubstrate at a growth temperature higher than 1,150° C. further reducesthe full width at half maximum of an XRD (004) rocking curve. Anotheradvantage is that the grown gallium nitride crystal can be preventedfrom being cracked in the polishing of the sliced piece after slicingmore effectively, thus further improving the yield in the polishing. Ata growth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes growthto form a thick film for a long time and limits the growth rate of thegallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. This invention canprevent cracking of the gallium nitride crystal during the growth, sothat the gallium nitride crystal can be reliably grown. It avoids localstresses due to surface irregularities of the seed crystal substrate.

In the methods according to the present invention, the second area ofthe seed crystal substrate more preferably has a surface roughness of 1μm or less in terms of arithmetic average roughness Ra. This inventioncan prevent cracking of the gallium nitride crystal during the growthmore effectively, so that the gallium nitride crystal can be morereliably grown.

In the methods according to the present invention, the first regions arecomposed of single-crystal gallium nitride, the second region iscomposed of single-crystal gallium nitride, and the crystal axis of thesingle-crystal gallium nitride in the first regions is opposite inorientation to that of the single-crystal gallium nitride in the secondregion.

According to this method, a gallium nitride crystal having a crystalaxis corresponding to that of the single-crystal gallium nitride in thesecond region is grown on the seed crystal substrate.

In the methods according to the present invention, the N-plane of thesingle-crystal gallium nitride can appear on the first areas, therecesses can be formed by etching the primary surface of the galliumnitride substrate in the step of forming the recesses in the first areasto prepare the seed crystal substrate, and the etching can be performedwith at least one of HCl, Cl₂, BCl₃, and CCl₄.

This method allows selective dry etching of the single-crystal galliumnitride in the first areas to prepare the seed crystal substrate becausethe crystal axis of the single-crystal gallium nitride in the firstregions is opposite in orientation to that of the single-crystal galliumnitride in the second region. Through the dry etching, the recesses areformed in the primary surface of the gallium nitride substrate.

In the methods according to the present invention, the N-plane of thesingle-crystal gallium nitride appears on the first areas, the recessesare formed by etching the primary surface of the gallium nitridesubstrate in the step of forming the recesses in the first areas toprepare the seed crystal substrate, and the etching is performed with asolution containing at least one of phosphoric acid, nitric acid, andsulfuric acid.

This method allows selective wet etching of the single-crystal galliumnitride in the first areas with the solution containing at least one ofphosphoric acid, nitric acid, and sulfuric acid to prepare the seedcrystal substrate because the crystal axis of the single-crystal galliumnitride in the first regions is opposite in orientation to that of thesingle-crystal gallium nitride in the second region. Through the wetetching, the recesses are formed in the primary surface of the galliumnitride substrate.

In the methods according to the present invention, the N-plane of thesingle-crystal gallium nitride appears on the first areas, the recessesare formed by etching the primary surface of the gallium nitridesubstrate in the step of forming the recesses in the first areas toprepare the seed crystal substrate, and the etching is performed with asolution containing potassium hydroxide.

This method allows selective wet etching of the single-crystal galliumnitride in the first areas with the solution containing potassiumhydroxide to prepare the seed crystal substrate because the crystal axisof the single-crystal gallium nitride in the first regions is oppositein orientation to that of the single-crystal gallium nitride in thesecond region. Through the wet etching, the recesses are formed in theprimary surface of the gallium nitride substrate.

In the methods according to the present invention, alternatively, theetching can be performed with a solution containing sodium hydroxide.Also, the etching can be performed with a solution containing at leastone of potassium hydroxide and sodium hydroxide. These etchants allowselective wet etching of the single-crystal gallium nitride in the firstareas, whereby the seed crystal substrate can be prepared.

In the methods according to the present invention, alternatively, theetching can be performed with a melt containing potassium hydroxide, orcan be performed with a melt containing sodium hydroxide. Also, theetching can be performed with a melt containing at least one ofpotassium hydroxide and sodium hydroxide. These melts can be used toperform desired etching within a shorter period of time than thatrequired for their solutions.

Another aspect of the present invention is a method for fabricating agallium nitride crystal. This method includes the steps of (a) preparinga gallium nitride substrate including a plurality of first regionshaving a dislocation density higher than a first dislocation density, asecond region having a dislocation density lower than the firstdislocation density, and a primary surface including first areas wherethe first regions appear and a second area where the second regionappears; (b) forming a mask over the first areas to prepare a seedcrystal substrate; and (c) growing gallium nitride on the galliumnitride of the seed crystal substrate by vapor-phase deposition to embedthe mask.

According to this method, the mask is embedded under the gallium nitridecrystal by selectively growing the gallium nitride crystal in the secondarea on the seed crystal substrate by vapor-phase deposition, so thatthe gallium nitride crystal is integrally grown on the second region.Accordingly, the gallium nitride crystal has a lower dislocation densitybecause no gallium nitride crystal inheriting dislocations from thefirst regions is formed.

Another aspect of the present invention is a method for fabricating agallium nitride crystal. This method includes the steps of (a) preparinga gallium nitride substrate including a plurality of first regionshaving a dislocation density higher than a first dislocation density, asecond region having a dislocation density lower than the firstdislocation density, and a primary surface including first areas wherethe first regions appear and a second area where the second regionappears; (b) forming a mask over the first areas to prepare a seedcrystal substrate; and (c) growing gallium nitride on the galliumnitride of the seed crystal substrate by liquid-phase deposition so asto embed the mask.

According to this method, the mask is embedded under the gallium nitridecrystal by selectively growing the gallium nitride crystal in the secondarea on the seed crystal substrate by liquid-phase deposition, so thatthe gallium nitride crystal is integrally grown on the second region.Accordingly, the gallium nitride crystal has a lower dislocation densitybecause no gallium nitride crystal inheriting dislocations from thefirst regions is formed.

Another aspect of the present invention is a method for fabricating agallium nitride crystal. This method includes the steps of (a) preparinga gallium nitride substrate including a plurality of first regionshaving a dislocation density higher than a first dislocation density, asecond region having a dislocation density lower than the firstdislocation density, and a primary surface including first areas wherethe first regions appear and a second area where the second regionappears; (b) preparing a seed crystal substrate by forming a mask so asto cover the first areas; and (c) growing a gallium nitride crystal onthe seed crystal substrate by vapor-phase deposition at a growthtemperature higher than 1,100° C. and equal to or lower than 1,300° C.to form a gallium nitride crystal thicker than the mask.

According to this method, the mask is embedded under the gallium nitridecrystal by selectively growing the gallium nitride crystal in the secondarea on the seed crystal substrate by vapor-phase deposition, so thatthe gallium nitride crystal is integrally grown on the second region.Accordingly, the gallium nitride crystal has a lower dislocation densitybecause gallium nitride crystal not inheriting dislocations from thefirst regions is formed. In addition, growing the gallium nitridecrystal on the seed crystal substrate at a growth temperature higherthan 1,100° C. reduces new dislocations occurring when the galliumnitride crystal grown in the second area is integrally grown over themask and also reduces the full width at half maximum of an XRD (004)rocking curve. Another advantage is that the grown gallium nitridecrystal can be prevented from being cracked during polishing afterslicing, thus improving the yield after the polishing. A growthtemperature higher than 1,100° C. reduces stress-concentrated sites inthe gallium nitride crystal to prevent the clacking during thepolishing. A growth temperature exceeding 1,300° C., on the other hand,causes damaging due to accelerated decomposition of the seed crystalsubstrate and a significant decrease in the growth rate of the galliumnitride crystal. It is generally believed that raising the growthtemperature increases the formation rate of a gallium nitride crystaland therefore increases its growth rate. It is, however, that thedecomposition rate of the gallium nitride crystal that has been formedincreases more significantly than the formation rate of the galliumnitride crystal at extraordinarily high growth temperatures, thusdecreasing the growth rate corresponding to the difference between theformation rate and the decomposition rate. Accordingly, extraordinarilyhigh growth temperatures are undesirable, and the growth temperature ispreferably equal to or lower than 1,300° C.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. and equal to or lower than 1,250° C.

According to this invention, growing the gallium nitride crystal on theseed crystal substrate at a growth temperature higher than 1,150° C.further reduces the full width at half maximum of an XRD (004) rockingcurve. Another advantage is that the grown gallium nitride crystal canbe prevented from being cracked during polishing of sliced pieceseffectively, thus further improving the yield after the polishing. At agrowth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes growthto form a thick film for a long time and limits the growth rate of thegallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.Although the reason is unclear, it may avoid local stresses due tosurface irregularities of the seed crystal substrate.

In the methods according to the present invention, the second area ofthe seed crystal substrate more preferably has a surface roughness of 1μm or less in terms of arithmetic average roughness Ra. This inventioncan prevent cracking of the gallium nitride crystal during the growthmore effectively, so that the gallium nitride crystal can be morereliably grown.

In the methods according to the present invention, the mask ispreferably composed of at least one of silicon oxide and siliconnitride. If the mask is composed of such a material, gallium nitridecrystal does not grow thereon, and high-quality gallium nitride growingon the second region grows in the transverse direction, finally formingintegrated gallium nitride.

If the mask is composed of such a material, no gallium nitride crystalgrows thereon, and a high-quality gallium nitride crystal growing on thesecond region grows in the lateral direction, finally forming integratedgallium nitride.

In the methods according to the present invention, each first region hasa stripe shape, and the second region is provided between the firstregions. In the methods according to the present invention,alternatively, the first regions are arranged in an array, and the firstregions are separated from each other by the second region.

A still another aspect of the present invention is a method forfabricating a gallium nitride crystal. This method includes the steps of(a) preparing a gallium nitride substrate including a plurality of firstregions having a dislocation density higher than a first dislocationdensity, a second region having a dislocation density lower than thefirst dislocation density, and a primary surface including first areaswhere the first regions appear and a second area where the second regionappears; (b) forming recesses in the first areas and a mask over therecesses in the first areas to prepare a seed crystal substrate; and (c)growing a gallium nitride crystal on the seed crystal substrate byliquid-phase deposition or vapor-phase deposition such that voidscorresponding to the recesses are formed.

According to this method, the mask is formed over the recesses in thefirst areas on the seed crystal substrate, and the gallium nitridecrystal is grown on the seed crystal substrate such that the voids areformed. The gallium nitride crystal is therefore integrally grown on thesecond region. Accordingly, the gallium nitride crystal has a lowerdislocation density because gallium nitride crystal not inheritingdislocations from the first regions is formed.

A still another aspect of the present invention is a method forfabricating a gallium nitride crystal. This method includes the steps of(a) preparing a gallium nitride substrate including a plurality of firstregions having a dislocation density higher than a first dislocationdensity, a second region having a dislocation density lower than thefirst dislocation density, and a primary surface including first areaswhere the first regions appear and a second area where the second regionappears; (b) forming recesses in the first areas and a mask over therecesses in the first areas to prepare a seed crystal substrate, therecesses being covered with the mask; and (c) growing a gallium nitridecrystal on the seed crystal substrate by vapor-phase deposition at agrowth temperature higher than 1,100° C. and equal to or lower than1,300° C.

According to this method, the mask is formed over the recesses in thefirst areas on the seed crystal substrate, and the gallium nitridecrystal is grown on the seed crystal substrate such that the voids areformed. The gallium nitride crystal is therefore integrally grown on thesecond region. Accordingly, the gallium nitride crystal has a lowerdislocation density because gallium nitride crystal not inheritingdislocations from the first regions is formed. In addition, growing thegallium nitride crystal on the seed crystal substrate at a growthtemperature higher than 1,100° C. reduces new dislocations occurringwhen the gallium nitride crystal grown in the second area is integrallygrown over the mask and also reduces the full width at half maximum ofan XRD (004) rocking curve. Another advantage is that the grown galliumnitride crystal can be prevented from being cracked during polishing ofsliced pieces, thus improving the yield after the polishing. A growthtemperature higher than 1,100° C. can reduces stress-concentrated sitesin the gallium nitride crystal to prevent cracking during the polishing.A growth temperature exceeding 1,300° C., on the other hand, causesdamaging due to accelerated decomposition of the seed crystal substrateand a significant decrease in the growth rate of the gallium nitridecrystal. It is generally believed that raising the growth temperatureincreases the formation rate of a gallium nitride crystal and thereforeincreases its growth rate. It is, however, that the decomposition rateof the gallium nitride crystal that has been formed increases moresignificantly than the formation rate of the gallium nitride crystal atextraordinarily high growth temperatures, thus decreasing the growthrate depending upon the difference between the formation rate and thedecomposition rate. Accordingly, extraordinarily high growthtemperatures are undesirable, and the growth temperature is preferablyequal to or lower than 1,300° C.

With the seed crystal substrate prepared by forming both the recessesand the mask in the first areas, gallium nitride crystal not inheritingthe first areas is grown even under such growth conditions that thegrowth rate reaches 200 μm/h or more. In contrast, the first areas canaffect the growth of a gallium nitride crystal on the following seedcrystal substrate under such growth conditions that the growth ratereaches 200 μm/h or more: a seed crystal substrate prepared by formingthe recesses in the first areas; a seed crystal substrate prepared byforming the mask in the first areas; and a seed crystal substrateprepared without forming the recesses or the mask in the first areas.Hence, the growth of the gallium nitride crystal on the seed crystalsubstrate having both the recesses and the mask has a cost advantageover the growth on the other seed crystal substrates.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. and equal to or lower than 1,250° C.According to this invention, growing the gallium nitride crystal on theseed crystal substrate at a growth temperature higher than 1,150° C.further reduces the full width at half maximum of an XRD (004) rockingcurve. Another advantage is that the grown gallium nitride crystal canbe prevented from being cracked during polishing of sliced pieces moreeffectively, thus further improving the yield of the polishing. At agrowth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes extendedgrowth to form a thick film for a long time and limits the growth rateof the gallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.It avoids local stresses due to surface irregularities of the seedcrystal substrate.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 1 μm orless in terms of arithmetic average roughness Ra. This invention canprevent cracking of the gallium nitride crystal during the growth moreeffectively, so that the gallium nitride crystal can be more reliablygrown.

In the methods according to the present invention, the first regions canbe composed of single-crystal gallium nitride, the second region can becomposed of single-crystal gallium nitride, and the crystal axis of thesingle-crystal gallium nitride in the first regions can be opposite inorientation to that of the single-crystal gallium nitride in the secondregion.

In the methods according to the present invention, preferably, theN-plane of the single-crystal gallium nitride appears on the firstareas, the recesses are formed by etching the primary surface of thegallium nitride substrate in the step of preparing the seed crystalsubstrate by forming the recesses in the first areas and forming themask so as to cover the recesses in the first areas, and the etching isperformed with at least one of HCl, Cl₂, BCl₃, and CCl₄. In the methodsaccording to the present invention, preferably, the N-plane of thesingle-crystal gallium nitride appears on the first areas, the recessesare formed by etching the primary surface of the gallium nitridesubstrate in the step of forming the recesses in the first areas and themask over the recesses in the first areas to prepare the seed crystalsubstrate, and the etching is performed with a solution containing atleast one of phosphoric acid, nitric acid, and sulfuric acid. In themethods according to the present invention, preferably, the N-plane ofthe single-crystal gallium nitride appears on the first areas, therecesses are formed by etching the primary surface of the galliumnitride substrate in the step of forming the recesses in the first areasand the mask over the recesses in the first areas to prepare the seedcrystal substrate, and the etching is performed with a solutioncontaining at least one of potassium hydroxide and sodium hydroxide. Inthe methods according to the present invention, the N-plane of thesingle-crystal gallium nitride appears on the first areas, the recessesare formed by etching the primary surface of the gallium nitridesubstrate in the step of preparing the seed crystal substrate by formingthe recesses in the first areas and forming the mask so as to cover therecesses in the first areas, and the etching is performed with a meltcontaining at least one of potassium hydroxide and sodium hydroxide.With such solutions and melts, the N-plane can be selectively etched.

In the methods according to the present invention, the mask ispreferably composed of at least one of silicon oxide and siliconnitride. If the mask is composed of such a material, gallium nitridecrystal does not grow thereon, and high-quality gallium nitride growingon the second region grows in the lateral direction, finally formingintegrated gallium nitride.

A further aspect of the present invention is a method for fabricating agallium nitride crystal. This method includes the steps of (a) preparinga gallium nitride substrate including a plurality of first regionshaving a dislocation density higher than a first dislocation density, asecond region having a dislocation density lower than the firstdislocation density, and a primary surface including first areas wherethe first regions appear and a second area where the second regionappears; and (b) growing a gallium nitride crystal using the galliumnitride substrate as a seed crystal substrate by vapor-phase depositionat a growth temperature higher than 1,100° C. and equal to or lower than1,300° C., neither recesses nor a mask being formed in the first areas.

According to this method, a growth temperature higher than 1,100° C.facilitates lateral growth of the gallium nitride crystal on the seedcrystal substrate to reduce the proportion of dislocations inheritedfrom the first regions and also reduces new dislocations occurring whenthe gallium nitride crystal is integrally grown in the transversedirection. In addition, this method can omit a surface-processing stepfor forming recesses or a mask on the seed crystal substrate becausethey are not required, and can prevent new dislocations from occurringdue to surface irregularities on the seed crystal substrate.Accordingly, the grown gallium nitride crystal can have a still lowerdislocation density. In addition, the full width at half maximum of anXRD (004) rocking curve can be reduced. Another advantage is that thegrown gallium nitride crystal can be prevented from being cracked duringpolishing after slicing, thus improving the yield after the polishing. Agrowth temperature higher than 1,100° C. will reduce stress-concentratedsites in the gallium nitride crystal to prevent the cracking during thepolishing. A growth temperature exceeding 1,300° C., on the other hand,causes damaging due to accelerated decomposition of the seed crystalsubstrate and a significant decrease in the growth rate of the galliumnitride crystal. It is generally believed that raising the growthtemperature increases the formation rate of a gallium nitride crystaland therefore increases its growth rate. It is assumed, however, thatthe decomposition rate of the gallium nitride crystal that has beenformed increases more significantly than the formation rate of thegallium nitride crystal at extraordinarily high growth temperatures,thus decreasing the growth rate depending upon the difference betweenthe formation rate and the decomposition rate. Accordingly,extraordinarily high growth temperatures are undesirable, and the growthtemperature is preferably equal to or lower than 1,300° C.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. In addition, the growth temperatureis preferably equal to or lower than 1,250° C. According to thisinvention, growing the gallium nitride crystal on the seed crystalsubstrate at a growth temperature higher than 1,150° C. further reducesthe full width at half maximum of an XRD (004) rocking curve. Anotheradvantage is that the grown gallium nitride crystal can be preventedfrom being cracked during polishing of slicing more effectively, thusfurther improving the yield in the polishing. At a growth temperatureexceeding 1,250° C., however, the seed crystal substrate decomposes tosome extent, though not so significantly, even though the temperaturedoes not exceed 1,300° C. This precludes growth to form a thick film fora long time, and limits the growth rate of the gallium nitride crystalto a certain level, resulting in cost disadvantage. Accordingly, thegrowth temperature is more preferably equal to or lower than 1,250° C.

In the method according to the present invention, the second area of theseed crystal substrate preferably has a surface roughness of 10 μm orless in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.It may avoid local stresses due to surface irregularities of the seedcrystal substrate.

In the method according to the present invention, the second area of theseed crystal substrate more preferably has a surface roughness of 1 μmor less in terms of arithmetic average roughness Ra. This invention canprevent cracking of the gallium nitride crystal during the growth moreeffectively, so that the gallium nitride crystal can be more reliablygrown.

In the method according to the present invention, the first regions arecomposed of single-crystal gallium nitride, the second region iscomposed of single-crystal gallium nitride, and the crystal axis of thesingle-crystal gallium nitride in the first regions has the sameorientation as that of the single-crystal gallium nitride in the secondregion.

According to this method, a gallium nitride crystal having a crystalaxis corresponding to that of the single-crystal gallium nitride in thefirst and second regions is grown on the seed crystal substrate.

In the methods according to the present invention, the seed crystalsubstrate is prepared by forming a gallium nitride substrate whichincludes the first regions formed by facet-growing a crystal such that agrowth surface thereof is not planar but has three-dimensional growthpits defined by facets and composites thereof and such that the growthpits and the composites thereof are not filled, whereby dislocations areconcentrated in the growth pits and the composites thereof.

In the methods according to the present invention, the seed crystalsubstrate is prepared by forming a gallium nitride substrate whichincludes the first regions formed by facet-growing a crystal on a basesubstrate having a mask having regularly striped pattern while linearV-grooves defined by facets are formed and maintained, wherebydislocations are concentrated in the bottoms of the V-grooves defined bythe facets.

In the methods according to the present invention, the seed crystalsubstrate is prepared by forming a gallium nitride substrate whichincludes the first regions formed by facet-growing a crystal on a basesubstrate having a regular seed pattern while pits defined by facets areformed and maintained, whereby dislocations are concentrated in thebottoms of the pits defined by the facets.

In the methods according to the present invention, the seed crystalsubstrate is a free-standing gallium nitride substrate with a thicknessof 100 μm or more.

In the methods according to the present invention, the seed crystalsubstrate is prepared to form a gallium nitride substrate having asurface that is the (0001) plane.

In the methods according to the present invention, the gallium nitridecrystal grown on the seed crystal substrate has a thickness of 200 μm ormore. The gallium nitride crystal grown on the seed crystal substrate bythis method is used to make a free-standing gallium nitride substrate.

The methods according to the present invention can further include thesteps of (d) separating the gallium nitride crystal from an integratedpiece of the gallium nitride crystal and the seed crystal substrate; and(e) forming a single-crystal gallium nitride wafer from the separatedgallium nitride crystal.

According to this method, a free-standing gallium nitride wafer isprepared from the separated gallium nitride crystal.

In the methods according to the present invention, the first regionspreferably have 1,000 or more dislocations per area of 10 μm square. Inthe methods according to the present invention, the first dislocationdensity is 1×10⁸ cm⁻² or more.

A still further aspect of the present invention is a single-crystalgallium nitride wafer fabricated by the above methods, and thedislocation density in a primary surface of the gallium nitride wafer is1×10⁶ cm⁻² or less. In addition, the number of dislocations per area of10 μm square in any region of a primary surface of a single-crystalgallium nitride wafer manufactured by the above methods is smaller thanthat in the first regions. In addition, the size of the gallium nitridewafers is 1 cm² or more. In addition, a single-crystal gallium nitridewafer is manufactured by the above methods, and the maximum dislocationdensity in a primary surface of the gallium nitride wafer is lower thanthe first dislocation density.

The present invention is not limited to the above aspects and also hasvarious aspects as described below. The technical contributions of theseaspects will be understood from the above description. A yet stillfurther aspect of the present invention is a method for fabricating agallium nitride crystal. This method includes the steps of (a) preparinga gallium nitride substrate including first regions composed ofsingle-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)preparing a seed crystal substrate by forming recesses in the firstareas; and (c) growing a gallium nitride crystal on the seed crystalsubstrate by liquid-phase deposition or vapor-phase deposition such thatvoids corresponding to the recesses are formed.

According to this method, the recesses are formed in the first areas onthe seed crystal substrate, and the gallium nitride crystal is grown onthe seed crystal substrate such that the voids are formed. The galliumnitride crystal is therefore integrally grown on the second region.Accordingly, gallium nitride crystal that does not inherit the firstregions, which have the crystal axis of opposite orientation, is notformed.

A yet still further aspect of the present invention is a method forfabricating a gallium nitride crystal. This method includes the steps of(a) preparing a gallium nitride substrate including first regionscomposed of single-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)preparing a seed crystal substrate by forming recesses in the firstareas; and (c) growing a gallium nitride crystal on the seed crystalsubstrate by vapor-phase deposition at a growth temperature higher than1,100° C. and equal to or lower than 1,300° C.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate by vapor-phase deposition such that the voidscorresponding to the recesses are formed while the gallium nitridecrystal is integrally grown on the second region. The gallium nitridecrystal is therefore grown not in the first areas but in the second areaon the seed crystal substrate. Accordingly, gallium nitride crystal notinheriting the crystal axis of opposite orientation from the firstregions is formed. In addition, growing the gallium nitride crystal onthe seed crystal substrate at a growth temperature higher than 1,100° C.reduces new dislocations occurring when the gallium nitride crystalgrown in the second area is integrally grown over the recesses and alsoreduces the full width at half maximum of an XRD (004) rocking curve.Another advantage is that the grown gallium nitride crystal can beprevented from being cracked during polishing after slicing, thusimproving the yield in the polishing. A growth temperature higher than1,100° C. may reduce stress-concentrated sites in the gallium nitridecrystal to prevent cracking during the polishing. A growth temperatureexceeding 1,300° C., on the other hand, causes damaging due toaccelerated decomposition of the seed crystal substrate and asignificant decrease in the growth rate of the gallium nitride crystal.It is generally believed that raising the growth temperature increasesthe formation rate of a gallium nitride crystal and therefore increasesits growth rate. It is, however, that the decomposition rate of thegallium nitride crystal that has been formed increases moresignificantly than the formation rate of the gallium nitride crystal atextraordinarily high growth temperatures, thus decreasing the net growthrate depending upon the difference between the formation rate and thedecomposition rate. Accordingly, extraordinarily high growthtemperatures are undesirable, and the growth temperature is preferablyequal to or lower than 1,300° C.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. and equal to or lower than 1,250° C.According to this invention, growing the gallium nitride crystal on theseed crystal substrate at a growth temperature higher than 1,150° C.further reduces the full width at half maximum of an XRD (004) rockingcurve. Another advantage is that the grown gallium nitride crystal canbe prevented from being cracked during polishing of sliced pieces moreeffectively, thus further improving the yield after the polishing. At agrowth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes growthto form a thick film for a long time and limits the growth rate of thegallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.It may avoid local stresses due to surface irregularities of the seedcrystal substrate. In the methods according to the present invention,additionally, the second area of the seed crystal substrate preferablyhas a surface roughness of 1 μm or less in terms of arithmetic averageroughness Ra. This invention can prevent cracking of the gallium nitridecrystal during the growth more effectively, so that the gallium nitridecrystal can be more reliably grown.

In the methods according to the present invention, preferably, theN-plane of the single-crystal gallium nitride appears on the firstareas, the recesses are formed by etching the primary surface of thegallium nitride substrate in the step of preparing the seed crystalsubstrate by forming the recesses in the first areas, and the etching isperformed with at least one of HCl, Cl₂, BCl₃, and CCl₄. In the methodaccording to the present invention, preferably, the N-plane of thesingle-crystal gallium nitride appears on the first areas, the recessesare formed by etching the primary surface of the gallium nitridesubstrate in the step of preparing the seed crystal substrate by formingthe recesses in the first areas, and the etching is performed with asolution containing at least one of phosphoric acid, nitric acid, andsulfuric acid. In the method according to the present invention,preferably, the N-plane of the single-crystal gallium nitride appears onthe first areas, the recesses are formed by etching the primary surfaceof the gallium nitride substrate in the step of forming the recesses inthe first areas to prepare the seed crystal substrate, and the etchingis performed with a solution containing at least one of potassiumhydroxide and sodium hydroxide. In the method according to the presentinvention, preferably, the N-plane of the single-crystal gallium nitrideappears on the first areas, the recesses are formed by etching theprimary surface of the gallium nitride substrate in the step ofpreparing the seed crystal substrate by forming the recesses in thefirst areas, and the etching is performed with a melt containing atleast one of potassium hydroxide and sodium hydroxide. With suchsolutions and melts, the N-plane can be selectively etched.

A method according to the present invention is a method for fabricatinga gallium nitride crystal. This method includes the steps of (a)preparing a gallium nitride substrate including first regions composedof single-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)forming a mask over the first areas to prepare a seed crystal substrate;and (c) growing gallium nitride on the seed crystal substrate byliquid-phase deposition or vapor-phase deposition so as to embed themask. The first area is covered with the mask.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate so as to embed the mask. Because the galliumnitride crystal is integrally grown on the second region, the galliumnitride crystal is grown not in the first areas but in the second areaon the seed crystal substrate. Accordingly, gallium nitride crystal notinheriting the crystal axis of opposite orientation from the firstregions is formed.

A method according to the present invention is a method for fabricatinga gallium nitride crystal. This method includes the steps of (a)preparing a gallium nitride substrate including first regions composedof single-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)forming a mask over the first areas to prepare a seed crystal substrate;and (c) growing a gallium nitride crystal on the seed crystal substrateby vapor-phase deposition at a growth temperature higher than 1,100° C.and equal to or lower than 1,300° C. to form a gallium nitride crystalthicker than the mask. The first area is covered with the mask.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. and equal to or lower than 1,250° C.According to this invention, growing the gallium nitride crystal on theseed crystal substrate at a growth temperature higher than 1,150° C.further reduces the full width at half maximum of an XRD (004) rockingcurve. Another advantage is that the grown gallium nitride crystal canbe prevented from being cracked during polishing of the slices moreeffectively, thus further improving the yield after the polishing. At agrowth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes growthto form a thick film for a long time and limits the growth rate of thegallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.It may avoid local stresses due to surface irregularities of the seedcrystal substrate. In the methods according to the present invention,additionally, the second area of the seed crystal substrate preferablyhas a surface roughness of 1 μm or less in terms of arithmetic averageroughness Ra. This invention can prevent cracking of the gallium nitridecrystal during the growth more effectively, so that the gallium nitridecrystal can be more reliably grown. In the methods according to thepresent invention, additionally, the mask is preferably composed of atleast one of silicon oxide and silicon nitride. If the mask is composedof such a material, gallium nitride crystal does not grow thereon, andhigh-quality gallium nitride growing on the second region grows in thelateral direction, finally forming integrated gallium nitride. In themethods according to the present invention, preferably, each firstregion has a striped pattern, and the second region is provided betweenthe first regions. In the methods according to the present invention,preferably, the first regions are arranged in an array, and the firstregions are separated from each other by the second region.

A method according to the present invention is a method for fabricatinga gallium nitride crystal. This method includes the steps of (a)preparing a gallium nitride substrate including first regions composedof single-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears;forming recesses in the first areas and a mask over the recesses in thefirst areas to prepare a seed crystal substrate; and (b) growing agallium nitride crystal on the seed crystal substrate by liquid-phasedeposition or vapor-phase deposition such that voids corresponding tothe recesses are formed. The mask covers the recesses.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate such that the voids are formed without fillingthe recesses with the GaN crystal and the gallium nitride crystal isintegrally grown on the second region. The gallium nitride crystal istherefore grown not in the first areas but in the second area on theseed crystal substrate. Accordingly, the gallium nitride crystal has alower dislocation density because gallium nitride crystal not inheritingthe crystal axis of opposite orientation from the first regions isformed.

A method according to the present invention is a method for fabricatinga gallium nitride crystal. This method includes the steps of (a)preparing a gallium nitride substrate including first regions composedof single-crystal gallium nitride, a second region composed ofsingle-crystal gallium nitride having a crystal axis opposite inorientation to that of the single-crystal gallium nitride in the firstregions, and a primary surface including first areas where the firstregions appear and a second area where the second region appears; (b)forming recesses in the first areas and a mask over the recesses in thefirst areas to prepare a seed crystal substrate; and (c) growing agallium nitride crystal on the seed crystal substrate by vapor-phasedeposition at a growth temperature higher than 1,100° C. and equal to orlower than 1,300° C. The recess es are covered with the mask.

According to this method, the gallium nitride crystal is grown on theseed crystal substrate by vapor-phase deposition such that the voids areformed from the recesses while the gallium nitride crystal is integrallygrown on the second region. The gallium nitride crystal is thereforegrown not in the first areas but in the second area on the seed crystalsubstrate. Accordingly, the gallium nitride crystal has a lowerdislocation density because gallium nitride crystal not inheriting thecrystal axis of opposite orientation from the first regions is formed.In addition, growing the gallium nitride crystal on the seed crystalsubstrate at a growth temperature higher than 1,100° C. reduces newdislocations occurring when the gallium nitride crystal grown in thesecond area is integrally grown over the recesses and also reduces thefull width at half maximum of an XRD (004) rocking curve. Anotheradvantage is that the grown gallium nitride crystal can be preventedfrom being cracked during polishing after slicing, thus improving theyield after the polishing. A growth temperature higher than 1,100° C.may reduce stress-concentrated sites in the gallium nitride crystal toprevent the cracking during the polishing. A growth temperatureexceeding 1,300° C., on the other hand, causes damaging due toaccelerated decomposition of the seed crystal substrate and asignificant decrease in the growth rate of the gallium nitride crystal.It is generally believed that raising the growth temperature increasesthe formation rate of a gallium nitride crystal and therefore increasesits growth rate. It is assumed, however, that the decomposition rate ofthe gallium nitride crystal that has been formed increases moresignificantly than the formation rate of the gallium nitride crystal atextraordinarily high growth temperatures, thus decreasing the growthrate depending upon the difference between the formation rate and thedecomposition rate. Accordingly, extraordinarily high growthtemperatures are undesirable, and the growth temperature is preferablyequal to or lower than 1,300° C.

In the method according to the present invention, the growth temperatureis preferably higher than 1,150° C. and equal to or lower than 1,250° C.According to this invention, growing the gallium nitride crystal on theseed crystal substrate at a growth temperature higher than 1,150° C.further reduces the full width at half maximum of an XRD (004) rockingcurve. Another advantage is that the grown gallium nitride crystal canbe prevented from being cracked during polishing of the slices moreeffectively, thus further improving the yield in the polishing. At agrowth temperature exceeding 1,250° C., however, the seed crystalsubstrate decomposes to some extent, though not so significantly, eventhough the temperature does not exceed 1,300° C. This precludes growthto form a thick film for a long time, and limits the growth rate of thegallium nitride crystal to a certain level, resulting in costdisadvantage. Accordingly, the growth temperature is more preferablyequal to or lower than 1,250° C.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 10 μmor less in terms of arithmetic average roughness Ra. According to thisinvention, the gallium nitride crystal is not cracked during the growth.It may avoid local stresses due to surface irregularities of the seedcrystal substrate.

In the methods according to the present invention, the second area ofthe seed crystal substrate preferably has a surface roughness of 1 μm orless in terms of arithmetic average roughness Ra. This invention canprevent cracking of the gallium nitride crystal during the growth moreeffectively, so that the gallium nitride crystal can be more reliablygrown.

In the methods according to the present invention, preferably, theN-plane of the single-crystal gallium nitride appears on the firstareas, the recesses are formed by etching the primary surface of thegallium nitride substrate in the step of forming the recesses in thefirst areas and the mask over the recesses in the first areas to preparethe seed crystal substrate, and the etching is performed with at leastone of HCl, Cl₂, BCl₃, and CCl₄. In the methods according to the presentinvention, preferably, the N-plane of the single-crystal gallium nitrideappears on the first areas, the recesses are formed by etching theprimary surface of the gallium nitride substrate in the step of formingthe recesses in the first areas and the mask over the recesses in thefirst areas to prepare the seed crystal substrate, and the etching isperformed with a solution containing at least one of phosphoric acid,nitric acid, and sulfuric acid. In the methods according to the presentinvention, preferably, the N-plane of the single-crystal gallium nitrideappears on the first areas, the recesses are formed by etching theprimary surface of the gallium nitride substrate in the step of formingthe recesses in the first areas and the mask so as to cover the recessesin the first areas to prepare the seed crystal substrate, and theetching is performed with a solution containing at least one ofpotassium hydroxide and sodium hydroxide. In the methods according tothe present invention, preferably, the N-plane of the single-crystalgallium nitride appears on the first areas, the recesses are formed byetching the primary surface of the gallium nitride substrate in the stepof forming the recesses in the first areas and the mask over therecesses in the first areas to prepare the seed crystal substrate, andthe etching is performed with a melt containing at least one ofpotassium hydroxide and sodium hydroxide. With such solutions and melts,the N-plane can be selectively etched.

In the methods according to the present invention, the mask ispreferably composed of at least one of silicon oxide and siliconnitride. If the mask is composed of such a material, no gallium nitridecrystal grows thereon, and high-quality gallium nitride growing on thesecond region grows in the lateral direction, finally forming integratedgallium nitride.

In the methods according to the present invention, the seed crystalsubstrate is preferably a free-standing gallium nitride substrate with athickness of 100 μm or more. In the methods according to the presentinvention, additionally, a surface of the seed crystal substrate ispreferably the (0001) plane. In the methods according to the presentinvention, additionally, the gallium nitride crystal grown on the seedcrystal substrate preferably has a thickness of 200 μm or more. Inaddition, the methods according to the present invention can furtherinclude the steps of separating the gallium nitride crystal from anintegrated piece of the gallium nitride crystal and the seed crystalsubstrate; and forming a single-crystal gallium nitride wafer from theseparated gallium nitride crystal.

A single-crystal gallium nitride wafer according to the presentinvention is a single-crystal gallium nitride wafer manufactured by theabove methods, and the crystal axis of the single-crystal galliumnitride has the same orientation in any region of the gallium nitridewafer. In addition, the size of the gallium nitride wafer according tothe present invention is 1 cm² or more.

The above and other objects, features, and advantages of the presentinvention will more easily be clarified from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

ADVANTAGES

The present invention, as described above, provides a method forfabricating a gallium nitride crystal with low dislocation density, highcrystallinity, and resistance to cracking during polishing of slicedpieces by growing the gallium nitride crystal on a seed crystalsubstrate including dislocation-concentrated regions orinverted-polarity regions, and also provides a gallium nitride wafer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes a process flowchart illustrating a method forfabricating a gallium nitride crystal according to a first embodimentand a diagram illustrating a gallium nitride wafer.

FIG. 2 includes a diagram illustrating a gallium nitride substrate, adiagram, taken from Box, of an example of the structure of the galliumnitride substrate, and a diagram, taken from Box, of another example ofthe structure of the gallium nitride substrate.

FIG. 3 includes a sectional view taken along line I-I of FIG. 2 and adiagram illustrating a step of the method for fabricating a galliumnitride crystal according to the first embodiment.

FIG. 4 includes diagrams illustrating steps of the method forfabricating a gallium nitride crystal according to the first embodiment.

FIG. 5 is a diagram illustrating the structure of a growth furnace usedin Examples.

FIG. 6 is a flowchart illustrating a method for fabricating a galliumnitride crystal according to a first embodiment.

FIG. 7 includes a sectional view taken along line I-I of FIG. 2 and adiagram illustrating a step of the method for fabricating a galliumnitride crystal according to the second embodiment.

FIG. 8 includes diagrams illustrating steps of the method forfabricating a gallium nitride crystal according to the secondembodiment.

FIG. 9 is a flowchart illustrating a method for fabricating a galliumnitride crystal according to a third embodiment.

FIG. 10 is a diagram of a gallium nitride seed crystal substrate used inthe method for fabricating a gallium nitride crystal according to thethird embodiment.

FIG. 11 includes diagrams illustrating steps of the method forfabricating a gallium nitride crystal according to the third embodiment.

FIG. 12 includes diagrams illustrating main steps of fabricating a seedcrystal substrate according to an additional embodiment.

REFERENCE NUMERALS

100 a, 100 b, and 100 c: process flow; 11: gallium nitride substrate;13: primary surface of gallium nitride substrate; 15: back surface ofgallium nitride substrate; 11 a: gallium nitride substrate; 17 a: firstregion; 19 a: second region; 15 a: back surface of gallium nitridesubstrate; 13 a: primary surface of gallium nitride substrate; 21 a:first area; 23 a: second area; 11 b: gallium nitride substrate; 13 b:primary surface of gallium nitride substrate; 15 b: back surface ofgallium nitride substrate; 17 b: first region; 19 b: second region; 21b: first area; 23 b: second area; 25: recess; 27: seed crystalsubstrate; 25 a: bottom surface of recess; 25 b and 25 c: side surfaceof recess; W1: opening width of recess; D1: depth of recess; D1/W1:aspect ratio of recess; 29: gallium nitride crystal; 31: void; H1:thickness of gallium nitride crystal; T_(G): growth temperature; S101 toS108: step; Wafer: gallium nitride wafer; 41: growth furnace; 43: growthchamber; 45: reaction tube; 47: support; 49 a: first line; 49 b: secondline; 53: gas unit; 51: boat; 55: heating unit; 57: seed crystalsubstrate; 58: mask; W2: width of first area; W3: width of mask; D2:thickness of mask; H2: thickness of gallium nitride crystal; 59: galliumnitride crystal; 63: separated gallium nitride crystal; 77: seed crystalsubstrate; 79: gallium nitride crystal; 83: separated gallium nitridecrystal

BEST MODES FOR CARRYING OUT THE INVENTION

The teachings of the present invention can readily be understood fromthe following detailed description with reference to the accompanieddrawings, which are presented for illustration. Embodiments of methodsfor fabricating a gallium nitride crystal and gallium nitride wafersaccording to the invention will now be described with reference to theattached drawings, where identified portions are denoted by identifiedreferences, if possible.

First Embodiment

With reference to FIG. 1, a method for fabricating a gallium nitridecrystal according to this embodiment will now be described. FIG. 1 showsa process flow 100 a. Part (A) of FIG. 2 is a diagram illustrating agallium nitride substrate. In Step S101, a gallium nitride substrate 11is prepared. In Part (A) of FIG. 2, the gallium nitride substrate 11 hasa primary surface 13 and a back surface 15. Part (B) of FIG. 2 is asectional view, taken from Box of Part (A) of FIG. 2, showing an exampleof the structure of the gallium nitride substrate. Part (C) of FIG. 2 isa sectional view, taken from Box of Part (A) of FIG. 2, showing anotherexample of the structure of the gallium nitride substrate. The Cartesiancoordinate system S is shown in Parts (B) and (C) of FIG. 2.

Referring to Part (B) of FIG. 2, a gallium nitride substrate 11 a isshown. The gallium nitride substrate 11 a includes a plurality of firstregions 17 a having a dislocation density higher than a predetermineddislocation density and second regions 19 a having a dislocation densitylower than the predetermined dislocation density. The predetermineddislocation density is, for example, 8×10⁷ cm⁻². The first regions 17 aare also called dislocation-concentrated regions. The first regions 17 aextend in one direction from a back surface 15 a to a primary surface 13a of the gallium nitride substrate 11 a, for example, parallel to the XYplane. The first regions 17 a extend in a striped pattern in the Ydirection. The second regions 19 a are separated by the first regions 17a. The first regions 17 a and the second regions 19 a are alternatelyarranged in the Z-axis direction. The primary surface 13 a of thegallium nitride substrate 11 a has first areas 21 a where the firstregions 17 a are exposed and second areas 23 a where the second regions19 a are exposed. The first areas 21 a and the second areas 23 a arealternately arranged in the Z-axis direction. Preferably, the firstareas 21 a and the second areas 23 a are regularly arranged.

Referring to Part (C) of FIG. 2, a gallium nitride substrate 11 b isshown. The gallium nitride substrate 11 b includes a plurality of firstregions 17 b having a dislocation density higher than a predetermineddislocation density and a second region 19 b having a dislocationdensity lower than the predetermined dislocation density. Thepredetermined dislocation density is, for example, 8×10⁷ cm⁻². The firstregions 17 b extend in one direction from a back surface 15 b to aprimary surface 13 b of the gallium nitride substrate 11 b, for example,in the X-axis direction. The first regions 17 b are, for example,regularly arranged in the Y-axis direction. The first regions 17 b arealso, for example, regularly arranged in the Z-axis direction. The firstregions 17 b are surrounded by the single second region 19 b. Theprimary surface 13 b of the gallium nitride substrate 11 b has firstareas 21 b where the first regions 17 b are exposed and a second area 23b where the second region 19 b are exposed. The first areas 21 b arealternately arranged in the Z-axis direction.

Parts (B) and (C) of FIG. 2 are merely illustrative of the structure ofthe gallium nitride substrate; the structure of the gallium nitridesubstrate applied to this embodiment is not limited to the specificstructures shown in the drawings.

In addition, an example of a gallium nitride substrate can include firstregions 17 a (17 b) composed of single-crystal gallium nitride andsecond regions 19 a (19 b) composed of single-crystal gallium nitride.The crystal axis of the single-crystal gallium nitride in the firstregions 17 a (17 b) is opposite in orientation to that of thesingle-crystal gallium nitride in the second regions 19 a (19 b).

Part (A) of FIG. 3 is a sectional view taken along line I-I of Part (B)of FIG. 2. A sectional view taken along line II-II of Part (C) of FIG. 2corresponds to the sectional view taken along line I-I of Part (B) ofFIG. 2. The subsequent description will focus on the gallium nitridesubstrate shown in Part (B) of FIG. 2. In Step S102 of FIG. 1, a seedcrystal substrate 27 shown in Part (B) of FIG. 3 is prepared by formingrecesses 25 in the first areas 21 a. In Part (B) of FIG. 3, the recesses25 extend in the Y-axis direction and have bottom surfaces 25 a and sidesurfaces 25 b and 25 c. The first regions 17 a are exposed in the bottomsurfaces 25 a. The second regions 17 a are exposed in the side surfaces25 b and 25 c. If the gallium nitride substrate 11 b shown in Part (C)of FIG. 2 is used, the recesses 25 are depressions in the primarysurface 13 b of the gallium nitride substrate 11 b, which are providedin an array. These depressions have bottom surfaces where the firstregions 17 b are exposed and side surfaces where the second region 17 ais exposed.

Next, examples of how the recesses 25 are formed will be described. In apreferred embodiment, the nitrogen (N) plane of the single-crystalgallium nitride is exposed on the first areas 21 a of the galliumnitride substrate 11 a, whereas the gallium (Ga) plane of thesingle-crystal gallium nitride is exposed on the second areas 23 a ofthe gallium nitride substrate 11 a; the first regions are also calledinverted-polarity regions.

According to a first forming method, the recesses 25 are formed bydry-etching the primary surface 13 a of the gallium nitride substrate 11a. The etching is performed with at least one gas of HCl, Cl₂, BCl₃, andCCl₄. The recesses are formed when the primary surface 13 a of thegallium nitride substrate 11 a is subjected to the etching gas becauseof a difference in etching rate between the first areas 21 a and thesecond areas 23 a. This method allows selective dry etching of thesingle-crystal gallium nitride in the first areas 21 a because thecrystal axis of the single-crystal gallium nitride in the first regions17 a is opposite in orientation to that of the single-crystal galliumnitride in the second regions 19 a. Through the dry etching, therecesses 25 are formed in the primary surface 13 a of the galliumnitride substrate 11 a to provide the seed crystal substrate 27 isprovided. The substrate temperature during the dry etching is preferably20° C. or higher because it will ensure an etching rate required to formthe recesses in the first areas 21 a. The substrate temperature, on theother hand, is preferably 900° C. or lower because a temperatureexceeding 900° C. would cause a significant etching damage to thesurface and thereby adversely affecting the quality of the crystal to begrown thereon.

According to a second forming method, the recesses 25 are formed bywet-etching the primary surface 13 a of the gallium nitride substrate 11a with an acid. The etching is performed with a solution containing atleast one of phosphoric acid, nitric acid, and sulfuric acid. Thismethod allows selective wet etching of the single-crystal galliumnitride in the first areas 21 a with a solution containing at least oneof phosphoric acid, nitric acid, and sulfuric acid because the crystalaxis of the single-crystal gallium nitride in the first regions 17 a isopposite in orientation to that of the single-crystal gallium nitride inthe second regions 19 a. Through the wet etching, the recesses 25 areformed in the primary surface 13 a of the gallium nitride substrate 11 ato prepare the seed crystal substrate 27.

According to a third forming method, the recesses 25 are formed bywet-etching the primary surface 13 a of the gallium nitride substrate 11a with an alkali. The etching is performed with a solution containingpotassium hydroxide. This method allows selective wet etching of thesingle-crystal gallium nitride in the first areas 21 a with a solutioncontaining potassium hydroxide because the crystal axis of thesingle-crystal gallium nitride in the first regions 17 a is opposite inorientation to that of the single-crystal gallium nitride in the secondregions 19 a. Through the wet etching, the recesses 25 are formed in theprimary surface 13 a of the gallium nitride substrate 11 a to preparethe seed crystal substrate 27.

Alternatively, the etching can be performed with a solution containingsodium hydroxide. Also, the etching can be performed with a solutioncontaining at least one of potassium hydroxide and sodium hydroxide.These etchants allow selective wet etching of the single-crystal galliumnitride in the first areas 21 a, whereby the seed crystal substrate 27can be prepared.

The recesses 25 are thus also formed on the primary surface 13 a of thegallium nitride substrate 11 a through wet etching with these etchants.

Alternatively, the etching can be performed with a melt containingpotassium hydroxide, or can be performed with a melt containing sodiumhydroxide. Also, the etching can be performed with a melt containing atleast one of potassium hydroxide and sodium hydroxide. These melts canbe used to perform desired etching within a shorter period of time thanthat required for their solutions.

In a preferred embodiment, the recesses 25 have an opening width W1 of,for example, 5 to 200 μm, a depth D1 of, for example, 10 μm or more, andan aspect ratio (D1/W1) of 2 or more.

In Step S103 of FIG. 1, after the recesses 25 are formed on the galliumnitride substrate 11 a, a gallium nitride crystal 29 is grown on theseed crystal substrate 27 by liquid-phase deposition or vapor-phasedeposition, as shown in Part (A) of FIG. 4. The gallium nitride crystal29 is grown on the seed crystal substrate 27 such that voids 31corresponding to the recesses 25 are formed. The gallium nitride crystal29 is a thick film with a thickness H1 of, for example, 200 μm or more.The gallium nitride crystal 29 grown on the seed crystal substrate 27 bythis method is used to manufacture a free-standing gallium nitridesubstrate.

According to this method, when the gallium nitride crystal 29 is grownon the seed crystal substrate 27 by liquid-phase deposition orvapor-phase deposition, the voids 31 associated to the recesses 25 areformed while gallium nitride crystals grown on the second regions 19 aare integrated into the gallium nitride crystal 29. The gallium nitridecrystal 29 is grown not in the first areas 21 a but in the second areas23 a on the seed crystal substrate 27. Accordingly, the gallium nitridecrystal 29 has a lower dislocation density because it does not inheritdislocations from the first regions 17 a.

In the vapor-phase deposition of the gallium nitride crystal 29 on theseed crystal substrate 27 having the recesses 25, as shown in Part (B)of FIG. 3, the growth temperature T_(G) is preferably higher than 1,100°C. This reduces additional dislocations occurring when the galliumnitride crystals grown on the second regions 17 b are integrated overthe recesses 25 without filling them, and also alleviates the effect ofvariations in the crystal quality of the second regions 17 b. The fullwidth at half maximum of an X-ray rocking curve XRD (004), which is atleast about 100 seconds for temperatures of not higher than 1,100° C.,falls below 100 seconds. The present embodiment has another advantagethat the grown gallium nitride crystal 29 can be prevented from beingcracked during polishing after slicing, thus improving the yield in thepolishing. A growth temperature higher than 1,100° C. can reducestress-concentrated sites in the gallium nitride crystal to preventcracking during the polishing. The yield after the polishing, which isabout 80% for temperatures of not higher than 1,100° C., increases to90% or more.

In addition, in the case of vapor-phase deposition of the galliumnitride crystal 29 on the seed crystal substrate 27 having the recesses25, as shown in Part (B) of FIG. 3, the growth temperature T_(G) ispreferably 1,300° C. or lower. A higher substrate temperature T_(G)causes damaging due to accelerated decomposition of the seed crystalsubstrate 27 and a significant decrease in the growth rate of thegallium nitride crystal 29. It is generally believed that raising thegrowth temperature increases the formation rate of a gallium nitridecrystal and therefore increases its growth rate. It is assumed, however,that the decomposition rate of the gallium nitride crystal that has beenformed increases more significantly than the formation rate of thegallium nitride crystal at extraordinarily high growth temperatures,thus decreasing the growth rate depending upon the difference betweenthe formation rate and the decomposition rate. If the temperatureexceeds 1,300° C., the growth rate will not be higher than 10 μm/h.Thus, if the growth temperature T_(G) is 1,300° C. or lower, a thickfilm of the gallium nitride crystal 29 can be formed within a practicalperiod of time with less damage to the seed crystal substrate 27.

In another embodiment, the growth temperature T_(G) is preferably higherthan 1,150° C. This reduces the full width at half maximum of an X-rayrocking curve XRD (004) to about 50 seconds and also increases the yieldafter the polishing to 95% or more. The growth temperature T_(G) ispreferably 1,250° C. or lower. At a growth temperature exceeding 1,250°C., the seed crystal substrate decomposes to some extent, though not sosignificantly, even though the temperature does not exceed 1,300° C.This precludes growth to from a thick film for a long time, and limitsthe growth rate of the gallium nitride crystal to a certain level,resulting in cost disadvantage. Accordingly, the growth temperature ispreferably 1,250° C. or lower. A temperature of 1,250° C. or lowerfacilitates the thick film growth and can increase the growth rate to 30μm/h or more, so that a thicker film of the gallium nitride crystal 29can be formed within a more practical period of time.

The gallium nitride crystal provided in this embodiment has improvedcrystal quality because the high-temperature growth prevents thegeneration of new dislocations and provides high crystallinity. Also,the gallium nitride crystal exhibits resistance to cracking duringpolishing after slicing.

In addition, the second areas 23 a (23 b) preferably have a surfaceroughness of 10 μm or less in terms of arithmetic average roughness Ra.In this case, cracking of the gallium nitride crystal 29, presumably dueto the surface roughness of the seed crystal substrate 27, can beprevented, so that the gallium nitride crystal 29 can be reliably grown.

More preferably, the second areas 23 a (23 b) more preferably have asurface roughness of 1 μm or less in terms of arithmetic averageroughness Ra. In this case, cracking of the gallium nitride crystal 29,presumably due to the surface roughness of the seed crystal substrate27, can be prevented more effectively, so that the gallium nitridecrystal 29 can be more reliably grown.

For the crystal growth, for example, a growth furnace applicable tohydride vapor-phase epitaxy is used. FIG. 5 is a diagram showing thegrowth furnace used in the examples to be described below. A growthfurnace 41 includes a reaction tube 45 for supplying gas into a growthchamber 43. A support 47 on which a seed crystal substrate is placed isdisposed in the reaction tube 45. The support 47 is provided with aheater for heating the substrate. Instead of providing the heater on thesupport 47, the substrate may be heated by raising the temperature ofthe support 47 by high-frequency induction heating using ahigh-frequency coil provided outside the reaction tube 45, and eithermethod is permitted. A first line 49 a and a second line 49 b extendfrom a gas unit 53 to the interior of the reaction tube 45. The firstline 49 a is connected to a hydrogen chloride source and a hydrogen gassource. The first line 49 a is connected to a boat 51 for a galliummaterial. Metallic gallium is placed in the boat 51. The second line 49b is connected to a hydrogen chloride source, an ammonia source, and ahydrogen gas source. The reaction tube 45 is surrounded by heating units55.

To grow a gallium nitride crystal, a mixed gas of hydrogen chloride andhydrogen gas is supplied from the first line 49 a. The hydrogen chloridereacts with the gallium to form gallium chloride. A mixed gas G1 ofgallium chloride and hydrogen is supplied into the reaction tube 45. Amixed gas G2 of ammonia and hydrogen gas is supplied from the secondline 49 b. These mixed gases form a gallium nitride crystal on thesubstrate in the reaction tube 45.

In a preferred embodiment, etching can be performed in the same furnaceto form recesses. In this case, hydrogen gas is supplied from the firstline 49 a. A mixed gas of hydrogen chloride and hydrogen gas is suppliedfrom the second line 49 b. These mixed gases selectively etch the firstregions of the gallium nitride substrate in the reaction tube 45. Afterthe etching, a gallium nitride crystal is grown.

In Step S104 of FIG. 1, subsequently, a gallium nitride crystal 33 isseparated from the integrated piece of the gallium nitride crystal 29and the seed crystal substrate 27. The gallium nitride crystal 33 can beseparated by slicing (cutting) or grinding. In subsequent Step S105 ofFIG. 1, a gallium nitride wafer is formed from the separated galliumnitride crystal 33. The separated gallium nitride crystal 33 is slicedinto a predetermined thickness and is mirror-polished. A damaged layerformed during the polishing is removed to complete the gallium nitridewafer. According to this method, a free-standing gallium nitride waferis prepared from the separated gallium nitride crystal. Part (B) of FIG.1 is a diagram showing the finished gallium nitride wafer. A galliumnitride wafer Wafer is made of single-crystal gallium nitride includingno dislocation-concentrated regions or inverted regions. The maximumdislocation density in a primary surface of the gallium nitride waferWafer is lower than a first dislocation density. The dislocation densityin the primary surface of the gallium nitride wafer Wafer is 1×10⁶ cm⁻²or less. In addition, the gallium nitride wafer has a size of an area of1 cm² or more. The full width at half maximum of an X-ray rocking curve(XRD) falls below 100 seconds.

The method used for the gallium nitride substrate 11 a according to theembodiment described above can also be used for a gallium nitridesubstrate having no or little difference in dislocation density betweenthe first regions 17 a and the second regions 19 a, for example, agallium nitride substrate including inverted-polarity regions wheredislocations are not concentrated. This method can make alow-dislocation-density gallium nitride crystal or gallium nitridesubstrate which does not inherit inverted-polarity regions and in whichdislocations are not generated in gallium nitride over theinverted-polarity regions, and can make crystals in high slicing yield.

The above gallium nitride wafer is used to manufacture an epitaxialsubstrate. The epitaxial substrate includes one or more gallium nitridesemiconductor films provided on the gallium nitride wafer. The galliumnitride wafer can be used as a substrate of, for example, opticaldevices (such as light-emitting diodes and laser diodes), electronicdevices (such as rectifiers, bipolar transistors, field-effecttransistors, and HEMTs), semiconductor sensors (such as temperaturesensors, pressure sensors, radiation sensors, and visible/ultravioletlight detectors), SAW devices, vibrators, resonators, oscillators, MEMScomponents, and piezoelectric actuators.

Example 1

In a growth furnace capable of locally heating a gallium nitridesubstrate, as shown in FIG. 5, a gallium nitride crystal was grown byHVPE. The gallium nitride substrate included dislocation-concentratedregions regularly arranged in a striped pattern andnon-dislocation-concentrated regions delimited by thedislocation-concentrated regions and had the structure shown in Part (B)of FIG. 2. The dislocation-concentrated regions are formed byfacet-growing the gallium nitride crystal while linear V-grooves definedby facets are formed and maintained, whereby dislocations wereconcentrated in the bottoms of the V-grooves defined by the facets. Thedislocation-concentrated regions had inverted-polarity regions, whereasthe non-dislocation-concentrated regions had uninverted-polarityregions. The gallium nitride substrate had a size of two inches and athickness of 400 μm, and its primary surface was the (0001) plane. Thegallium nitride substrate was placed on a support in a reaction tube.The substrate temperature was raised to 800° C., and theinverted-polarity regions of the gallium nitride substrate wereselectively etched by supplying HCl gas. Through the etching, a seedcrystal substrate was prepared. The seed crystal substrate had thearrangement of grooves formed by transferring the pattern of theinverted-polarity regions thereto. Subsequently, the substratetemperature was set to 1,200° C. The substrate temperature wasdetermined from the results of a temperature measurement carried out inadvance by use of a thermocouple attached to an alumina substrate placedon the support and. The alumina substrate had a thickness of 1 mm and asize of 2 inches. The substrate temperature was set to 1,200° C. byadjusting the substrate heater to the temperature conditions in whichthe thermocouple on the alumina substrate read 1,200° C. Galliumchloride was produced by heating a gallium boat filled with gallium to800° C. while supplying HCl gas and H₂ gas. A gallium nitride crystalwas grown into a thickness of 400 μm on the seed crystal substrate bysupplying the gallium chloride gas and ammonia gas. The partialpressures of HCl and NH₃ were adjusted such that the growth rate reached50 μm/h or more without polycrystal formation. A crystal qualityanalysis of the resulting gallium nitride crystal by X-raydiffractometry (XRD) showed that the full width at half maximum of arocking curve for the (004) plane was 50 seconds, which is an excellentlevel. Then, the surface of the formed gallium nitride crystal wasetched with a KOH—NaOH mixed melt at 350° C. The dislocation density wasevaluated by counting the number of etch pits on the etched surface;they are formed at positions corresponding to dislocations. Thedislocation density was 5×10⁵ cm⁻², meaning that alow-dislocation-density gallium nitride crystal was grown. When thegallium nitride crystal was immersed in a melt containing potassiumhydroxide, no recesses corresponding to inverted-polarity regions wereformed, meaning that a crystal containing no inverted phase was grown.

Example 2

A gallium nitride crystal was grown under the same conditions as thoseused in Example 1 except that the crystal was grown into a thickness of10 mm. The resultant gallium nitride crystal was sliced in a directionparallel to the surface of the seed-crystal substrate to form galliumnitride slices, and these slices were mirror-polished, thus preparingten gallium nitride crystal substrates with a thickness of 400 μm. Noneof the ten substrates was cracked during the polishing; the polishing,yield was 100%. One of the ten substrates was picked out and etched witha KOH—NaOH mixed melt at 350° C. The dislocation density was evaluatedby counting the number of etch pits on the etched surface; they appearat positions corresponding to dislocations. The dislocation density was5×10⁵ cm⁻², meaning that a low-dislocation-density gallium nitridecrystal was obtained. A crystal quality analysis of the gallium nitridecrystal by X-ray diffractometry (XRD) showed that the full width at halfmaximum of a rocking curve for the (004) plane was 50 seconds, which isan excellent level. When the gallium nitride crystal was immersed in amelt containing potassium hydroxide, no recesses corresponding toinverted-polarity regions were formed, meaning that a crystal withoutinverted-polarity phase was obtained.

Example 3

A gallium nitride substrate of the same type as used in Example 1 wasprepared. The inverted-polarity regions of the gallium nitride substratewere selectively etched with plasma by introducing BCl₃ gas into areactive ion etching (RIE) system. Through the etching, a seed crystalsubstrate was prepared. The seed crystal substrate had the arrangementof grooves formed by transferring the pattern for the inverted-polarityregions thereto. Subsequently, a gallium nitride crystal was grown underthe same conditions as those used in Example 1. The surface of theresultant gallium nitride crystal was etched with a KOH—NaOH mixed meltat 350° C. The dislocation density was evaluated by counting the numberof etch pits on the etched surface; they appear at positionscorresponding to dislocations. The dislocation density was 5×10⁵ cm⁻²,meaning that a low-dislocation-density gallium nitride crystal wasgrown. A crystal quality analysis of the gallium nitride crystal byX-ray diffractometry (XRD) showed that the full width at half maximum ofa rocking curve for the (004) plane was 50 seconds, which is anexcellent level. When the gallium nitride crystal was immersed in a meltcontaining potassium hydroxide, recesses corresponding toinverted-polarity regions were not formed, meaning that a crystalcontaining no inverted phase was grown.

Example 4

A gallium nitride substrate of the same type as used in Example 1 wasprepared. The inverted-polarity regions of the gallium nitride substratewere selectively etched with a mixed acid of phosphoric acid andsulfuric acid (the ratio is phosphoric acid:hydrochloric acid=1:1).Through the etching, a seed crystal substrate was prepared. The seedcrystal substrate had grooves corresponding to the pattern of theinverted-polarity regions. Subsequently, a gallium nitride crystal wasgrown under the same conditions as those used in Example 1. The surfaceof the resultant gallium nitride crystal was etched with a KOH—NaOHmixed melt at 350° C. The dislocation density was evaluated by countingthe number of etch pits on the etched surface; they appear at positionscorresponding to dislocations. The dislocation density was 5×10⁵ cm⁻²,meaning that a low-dislocation-density gallium nitride crystal wasobtained. A crystal quality analysis of the gallium nitride crystal byX-ray diffractometry (XRD) showed that the full width at half maximum ofa rocking curve for the (004) plane was 50 seconds, which is anexcellent level. When the gallium nitride crystal was immersed in a meltcontaining potassium hydroxide, no recesses corresponding to invertedregions were formed, meaning that a crystal containing no inverted phasewas obtained.

Comparative Example

A gallium nitride substrate of the same type as used in Example 1 wasprepared, and a seed crystal substrate was prepared. Subsequently, agallium nitride crystal was grown on the seed crystal substrate at asubstrate temperature of 1,000° C. (the growth conditions other than thetemperature were the same as those in Example 1). The surface of theresultant gallium nitride crystal was etched with a KOH—NaOH mixed meltat 350° C. The dislocation density was evaluated by counting the numberof etch pits on the etched surface; they appear at positionscorresponding to dislocations. The dislocation density was 1×10⁶ cm⁻²,meaning that a low-dislocation-density gallium nitride crystal wasobtained. A crystal quality analysis of the gallium nitride crystal byX-ray diffractometry (XRD) showed that the full width at half maximum ofa rocking curve for the (004) plane was 120 seconds.

According to this embodiment, a gallium nitride crystal with lowdislocation density, high crystallinity, and resistance to crackingduring polishing of sliced pieces is provided because the galliumnitride crystal is grown at high temperature.

Second Embodiment

With reference to FIG. 6, a method for fabricating a gallium nitridecrystal according to this embodiment will now be described. FIG. 6 showsa process flow 100 b. Part (A) of FIG. 2 is a diagram illustrating agallium nitride substrate. In Step S101, a gallium nitride substrate 11is prepared. As in the first embodiment, the gallium nitride substrate11 used may be either of the types shown in Parts (B) and (C) of FIG. 2.Also, in this embodiment, Parts (B) and (C) of FIG. 2 are merelyillustrative of the structure of the gallium nitride substrate; thestructure of the gallium nitride substrate applied to this embodiment isnot limited to the specific structures shown in the drawings.

Part (A) of FIG. 7 is a sectional view taken along line I-I of Part (B)of FIG. 2. A sectional view taken along line II-II of Part (C) of FIG. 2corresponds to the sectional view taken along line I-I of Part (B) ofFIG. 2. The subsequent description will focus on the gallium nitridesubstrate shown in Part (B) of FIG. 2. In Step S106 of FIG. 6, a mask 58is formed so as to cover the first areas 21 a to prepare a seed crystalsubstrate 57 shown in Part (B) of FIG. 7. The actual pattern of the mask58 is determined depending on the pattern of the first areas 21 a. InPart (B) of FIG. 7, the mask 58 extends in the Y-axis direction andcovers the first areas 21 a and parts of the second areas 23 a, whichextend along the first areas 21 a. The mask 58 is formed so that thefirst regions 17 a are not exposed. The second areas 23 a, composed ofgallium nitride, and the mask 58, composed of a material different fromgallium nitride, appear alternately in the primary surface of the seedcrystal substrate 57. The mask 58 is preferably composed of at least oneof silicon oxide and silicon nitride. If the mask 58 is composed of sucha material, no gallium nitride crystal grows thereon, and high-qualitygallium nitride crystals growing on the second regions 19 a grow in thetransverse direction, finally forming an integrated gallium nitridecrystal.

In a preferred embodiment, the first areas 21 a have a width W2 of, forexample, 5 to 200 and the mask 58 has a width W3 of, for example, 10 to250 μm and a thickness D2 of, for example, 5 to 20 μm.

In Step S107 of FIG. 6, after the mask 58 is formed on the galliumnitride substrate 11 a, a gallium nitride crystal 59 is grown on theseed crystal substrate 57 by liquid-phase deposition or vapor-phasedeposition, as shown in Part (A) of FIG. 8. The gallium nitride crystal59 is grown on the seed crystal substrate 57 so as to cover the mask 58.The gallium nitride crystal 59 is made of a thick film with a thicknessH2 similar to, for example, the thickness H1 of the gallium nitridecrystal 29. The gallium nitride crystal 59 grown on the seed crystalsubstrate 57 by this method is used to manufacture a free-standinggallium nitride substrate.

According to this method, the mask 58 is embedded under the galliumnitride crystal 59 by selectively growing gallium nitride crystals inthe second areas 23 a on the seed crystal substrate 57 by liquid-phasedeposition or vapor-phase deposition, so that the gallium nitridecrystals grown on the second regions 19 a are integrated. Accordingly,the gallium nitride crystal 59 has a lower dislocation density becauseno gallium nitride crystal inheriting dislocations from the firstregions 17 a is formed.

In the case of vapor-phase deposition of the gallium nitride crystal 59on the seed crystal substrate 57 having the mask, as shown in Part (B)of FIG. 7, the growth temperature T_(G) is preferably higher than 1,100°C. This reduces new dislocations occurring when the gallium nitridecrystals grown on the second regions 19 a are integrated over the mask58 and also alleviates the effect of variations in the crystal qualityof the second regions 19 a. The full width at half maximum of an X-rayrocking curve XRD (004), which is at least about 100 seconds fortemperatures of not higher than 1,100° C., falls below 100 seconds.Another advantage is that the grown gallium nitride crystal 59 can beprevented from being cracked during polishing after slicing, thusimproving the yield after the polishing. A growth temperature higherthan 1,100° C. may reduce stress-concentrated sites in the galliumnitride crystal to prevent the cracking during the polishing. The yieldafter the polishing, which is about 80% for temperatures of not higherthan 1,100° C., increases to 90% or more.

In addition, in vapor-phase deposition of the gallium nitride crystal 59on the seed crystal substrate 57 having the mask, as shown in Part (B)of FIG. 7, a higher substrate temperature causes damaging due toaccelerated decomposition of the seed crystal substrate 57 and asignificant decrease in the growth rate of the gallium nitride crystal59. It is generally believed that raising the growth temperatureincreases the formation rate of a gallium nitride crystal and thereforeincreases its growth rate. It is assumed, however, that thedecomposition rate of the gallium nitride crystal that has been formedincreases more significantly than the formation rate of the galliumnitride crystal at extraordinarily high growth temperatures, thusdecreasing the growth rate depending upon the difference between theformation rate and the decomposition rate. If the temperature exceeds1,300° C., the growth rate will not be higher than 10 μm/h. Thus, if thegrowth temperature T_(G) is 1,300° C. or lower, a thick film of thegallium nitride crystal 59 can be formed within a practical period oftime with less damage to the seed crystal substrate 57.

In another embodiment, the growth temperature T_(G) is preferably higherthan 1,150° C. This reduces the full width at half maximum of an X-rayrocking curve XRD (004) to about 50 seconds and also increases the yieldin steps after the polishing to 95% or more. The growth temperatureT_(G) is preferably 1,250° C. or lower. At a growth temperatureexceeding 1,250° C., the seed crystal substrate decomposes to someextent, though not so significantly, even though the temperature doesnot exceed 1,300° C. This precludes growth to form a thick film for along time and limits the growth rate of the gallium nitride crystal to acertain level, resulting in cost disadvantage. Accordingly, the growthtemperature is preferably 1,250° C. or lower. A temperature of 1,250° C.or lower facilitates the thick film growth and can increase the growthrate to 30 μm/h or more, so that a thicker film of the gallium nitridecrystal 59 can be formed within a more practical period of time. If thegallium nitride crystal 59 is grown below the growth temperatureaccording to the present invention, on the other hand, the masksignificantly affects the crystal growth; the gallium nitride crystalgrown over the mask has low crystallinity because the mask is composedof a different material.

The gallium nitride crystal provided in this embodiment has improvedcrystal quality because the high-temperature growth prevents newdislocations from occurring and provides high crystallinity. Also, thegallium nitride crystal has resistance to cracking during polishing ofsliced pieces.

In addition, the second areas 23 a preferably have a surface roughnessof 10 μm or less in terms of arithmetic average roughness Ra. In thiscase, cracking of the gallium nitride crystal 59, presumably due to thesurface roughness of the seed crystal substrate 57, can be prevented, sothat the gallium nitride crystal 59 can be reliably grown.

More preferably, the second areas 23 a have a surface roughness of 1 μmor less in terms of arithmetic average roughness Ra. In this case,cracking of the gallium nitride crystal 59, presumably due to thesurface roughness of the seed crystal substrate 57, can be preventedmore effectively, so that the gallium nitride crystal 59 can be morereliably grown.

For the crystal growth, for example, the growth furnace applicable tohydride vapor-phase epitaxy which is shown in FIG. 5 is used, as in thefirst embodiment.

In Step S104 of FIG. 6, subsequently, a gallium nitride crystal 63 isseparated from the integrated piece of the gallium nitride crystal 59and the seed crystal substrate 57, as shown in Part (B) of FIG. 8. Thegallium nitride crystal 63 can be separated by the same method as usedin the first embodiment. In Step S105 of FIG. 6, subsequently, a galliumnitride wafer is formed from the separated gallium nitride crystal 63.The separated gallium nitride crystal 63 is sliced into a predeterminedthickness and is mirror-polished. A damaged layer formed during thepolishing is removed to complete the gallium nitride wafer.

According to this method, a free-standing gallium nitride wafer isprepared from the separated gallium nitride crystal. Thus, a galliumnitride wafer as shown in Part (B) of FIG. 1 is completed. A galliumnitride wafer Wafer according to this embodiment is a single-crystalgallium nitride wafer including no dislocation-concentrated regions orinverted regions. The maximum dislocation density in a primary surfaceof the gallium nitride wafer Wafer is lower than a first dislocationdensity. The maximum dislocation density in a primary surface of thegallium nitride wafer Wafer is lower than a first dislocation density.The dislocation density in the primary surface of the gallium nitridewafer Wafer is 1×10⁶ cm⁻² or less. In addition, the gallium nitridewafer has a size of an area of 1 cm² or more. The full width at halfmaximum of an X-ray rocking curve (XRD) falls below 100 seconds.

The method used for the gallium nitride substrate 11 a according to theembodiment described above can also be used for a gallium nitridesubstrate having no or little difference in dislocation density betweenthe first regions 17 a and the second regions 19 a, for example, agallium nitride substrate including inverted-polarity regions where nodislocations are concentrated. This method can make alow-dislocation-density gallium nitride crystal or gallium nitridesubstrate which does not inherit inverted regions and in which nodislocations occur over the inverted regions, and can make crystals inhigh slicing yield.

The above gallium nitride wafer is used to fabricate an epitaxialsubstrate. The epitaxial substrate includes one or more gallium nitridesemiconductor films provided on the gallium nitride wafer. The galliumnitride wafer can be used as a substrate of, for example, opticaldevices (such as light-emitting diodes and laser diodes), electronicdevices (such as rectifiers, bipolar transistors, field-effecttransistors, and HEMTs), semiconductor sensors (such as temperaturesensors, pressure sensors, radiation sensors, and visible/ultravioletlight detectors), SAW devices, vibrators, resonators, oscillators, MEMScomponents, and piezoelectric actuators.

Example 5

A gallium nitride substrate similar to that used in Example 1 wasprepared. A silicon nitride mask was formed by photolithography andetching. This mask covered dislocation-concentrated regions of thegallium nitride substrate. Subsequently, a gallium nitride crystal wasgrown under the same conditions as those used in Example 1. The surfaceof the resultant gallium nitride crystal was etched with a KOH—NaOHmixed melt at 350° C. The dislocation density was evaluated by countingthe number of etch pits on the etched surface; they appear at positionscorresponding to dislocations. The dislocation density was 5×10⁵ cm⁻²,meaning that a low-dislocation-density gallium nitride crystal wasgrown. A crystal quality analysis of the gallium nitride crystal byX-ray diffractometry (XRD) showed that the full width at half maximum ofa rocking curve for the (004) plane was 50 seconds, which is anexcellent level. When the gallium nitride crystal was immersed in a meltcontaining potassium hydroxide, the observation of the surface of thegallium nitride shows that recesses corresponding to inverted-polarityregions were not formed, meaning that a crystal containing no invertedphase was grown.

Example 6

A gallium nitride crystal was grown under the same conditions as thoseused in Example 5 except that the crystal was grown into a thickness of10 mm. The resultant gallium nitride crystal was sliced in a directionparallel to the surface of the seed-crystal substrate, and wasmirror-polished, thus preparing ten gallium nitride crystal substrateswith a thickness of 400 μm. None of the ten substrates was crackedduring the polishing; the polishing yield was 100%. One of the tensubstrates was picked out and etched with a KOH—NaOH mixed melt at 350°C. The dislocation density was evaluated by counting the number of etchpits on the etched surface; they appear at positions corresponding todislocations. The dislocation density was 5×10⁵ cm⁻², meaning that alow-dislocation-density gallium nitride crystal was grown. A crystalquality analysis of the gallium nitride crystal by X-ray diffractometry(XRD) showed that the full width at half maximum of a rocking curve forthe (004) plane was 50 seconds, which is an excellent level. When thegallium nitride crystal was immersed in a melt containing potassiumhydroxide, the observation of the surface of the gallium nitride showsthat recesses corresponding to inverted-polarity regions were notformed, meaning that a crystal containing no inverted phase was grown.

According to this embodiment, a gallium nitride crystal with lowdislocation density, high crystallinity, and resistance to crackingduring polishing of the sliced pieces slicing is provided because thegallium nitride crystal is grown at high temperature.

Third Embodiment

With reference to FIG. 9, a method for fabricating a gallium nitridecrystal according to this embodiment will now be described.

FIG. 9 shows a process flow 100 b. Part (A) of FIG. 2 is a diagramillustrating a gallium nitride substrate. In Step S101, a galliumnitride substrate 11 is prepared. As in the first embodiment, thegallium nitride substrate 11 used may be either of the types shown inParts (B) and (C) of FIG. 2. Also, in this embodiment, Parts (B) and (C)of FIG. 2 are merely illustrative of the structure of the galliumnitride substrate; the structure of the gallium nitride substrateapplied to this embodiment is not limited to the specific structuresshown in the drawings.

FIG. 10 is a sectional view taken along line I-I of Part (B) of FIG. 2.A sectional view taken along line II-II of Part (C) of FIG. 2corresponds to the sectional view taken along line I-I of Part (B) ofFIG. 2. The subsequent description will focus on the gallium nitridesubstrate shown in Part (B) of FIG. 2.

In Step S108 of FIG. 9, a gallium nitride crystal 79 is grown on a seedcrystal substrate 77 by vapor-phase deposition without forming recessesor a mask on the gallium nitride substrate 11 a, as shown in Part (A) ofFIG. 11. The gallium nitride crystal 79 is made of a thick film with athickness H3 similar to, for example, the thickness H1 of the galliumnitride crystal 29. The gallium nitride crystal 79 grown on the seedcrystal substrate 77 by this method is used to fabricate a free-standinggallium nitride substrate.

In the case of vapor-phase deposition of the gallium nitride crystal 79on the seed crystal substrate 77 having no recesses or a mask, as shownin FIG. 10, the growth temperature T_(G) is higher than 1,100° C. Atemperature higher than 1,100° C. facilitates lateral growth of galliumnitride crystals on the second regions of the seed crystal substrate 77to alleviate the effect of the first regions of the seed crystalsubstrate 77. This reduces the proportion of dislocations inherited fromthe first regions, thus reducing the dislocation density. Such a hightemperature also reduces new dislocations occurring when the galliumnitride crystals grown in the lateral direction are integrated andalleviates the effect of variations in the crystal quality of the secondregions. The full width at half maximum of an X-ray rocking curve XRD(004), which is at least about 100 seconds for temperatures of nothigher than 1,100° C., falls below 100 seconds. Another advantage isthat the grown gallium nitride crystal 79 can be prevented from beingcracked during polishing after slicing, thus improving the yield afterthe polishing. A growth temperature higher than 1,100° C. may reducestress-concentrated sites in the gallium nitride crystal to prevent thecracking during the polishing. The yield after the polishing, which isabout 80% for temperatures of not higher than 1,100° C., increases to90% or more.

In addition, in vapor-phase deposition of the gallium nitride crystal 79on the seed crystal substrate 77 having no recesses or a mask, as shownin FIG. 10, the growth temperature T_(G) is preferably 1,300° C. orlower. A higher substrate temperature T_(G) causes damaging due toaccelerated decomposition of the seed crystal substrate 77 and asignificant decrease in the growth rate of the gallium nitride crystal79. It is generally believed that raising the growth temperatureincreases the formation rate of a gallium nitride crystal and thereforeincreases its growth rate. It is assumed, however, that thedecomposition rate of the gallium nitride crystal that has been formedincreases more significantly than the formation rate of the galliumnitride crystal at extraordinarily high growth temperatures, thusdecreasing the growth rate corresponding to the difference between theformation rate and the decomposition rate. If the temperature exceeds1,300° C., the growth rate will not be higher than 10 μm/h. Thus, if thegrowth temperature T_(G) is 1,300° C. or lower, a thick film of thegallium nitride crystal 79 can be formed within a practical period oftime with less damage to the seed crystal substrate 77.

In one embodiment, the growth temperature T_(G) is preferably higherthan 1,150° C. This reduces the full width at half maximum of an X-rayrocking curve XRD (004) to about 50 seconds and also increases the yieldafter the polishing to 95% or more. The growth temperature T_(G) ispreferably 1,250° C. or lower. At a growth temperature exceeding 1,250°C., the seed crystal substrate decomposes to some extent, though not sosignificantly, even though the temperature does not exceed 1,300° C.This precludes growth to form a thick film for a long time, and limitsthe growth rate of the gallium nitride crystal to a certain level,resulting in cost disadvantage. Accordingly, the growth temperature ispreferably 1,250° C. or lower. A temperature of 1,250° C. or lowerfacilitates the thick film growth and can increase the growth rate to 30μm/h or more, so that a thicker film of the gallium nitride crystal 79can be formed within a more practical period of time.

This method, which does not form any recesses or mask on the seedcrystal substrate, is preferable to those of the first and secondembodiments because it can omit a surface-processing step for formingrecesses or a mask as well as prevent new dislocations from occurringdue to surface irregularities on the seed crystal substrate. Also, thegallium nitride crystal exhibits resistance to cracking during polishingof sliced pieces.

In addition, the seed crystal substrate 77 preferably has a surfaceroughness of 10 μm/h or less in terms of arithmetic average roughnessRa. In this case, cracking of the gallium nitride crystal 79, presumablydue to the surface roughness of the seed crystal substrate 77, can beprevented, so that the gallium nitride crystal 79 can be reliably grown.

More preferably, the seed crystal substrate 77 has a surface roughnessof 1 μM or less in terms of arithmetic average roughness Ra. In thiscase, cracking of the gallium nitride crystal 79, presumably due to thesurface roughness of the seed crystal substrate 77, can be preventedmore effectively, so that the gallium nitride crystal 79 can be morereliably grown.

For the crystal growth, for example, the growth furnace applicable tohydride vapor-phase epitaxy which is shown in FIG. 5 is used, as in thefirst embodiment.

In Step S104 of FIG. 9, subsequently, a gallium nitride crystal 83 isseparated from the integrated piece of the gallium nitride crystal 79and the seed crystal substrate 77. The gallium nitride crystal 83 can beseparated by the same method as used in the first embodiment. In StepS105 of FIG. 9, subsequently, a gallium nitride wafer is formed from theseparated gallium nitride crystal 83. The separate gallium nitridecrystal 83 is sliced into a predetermined thickness, and the slicedpieces are mirror-polished. A damaged layer formed during the polishingis removed to complete the gallium nitride wafer.

According to this method, a free-standing gallium nitride wafer isprepared from the separated gallium nitride crystal. Thus, a galliumnitride wafer as shown in Part (B) of FIG. 1 is completed. A galliumnitride wafer, Wafer, according to this embodiment is made of asingle-crystal gallium nitride including neitherdislocation-concentrated regions nor inverted-polarity regions. Themaximum dislocation density in a primary surface of the gallium nitridewafer, Wafer, is lower than a first dislocation density. The maximumdislocation density in a primary surface of the gallium nitride wafer,Wafer, is lower than a first dislocation density. The dislocationdensity in the primary surface of the gallium nitride wafer Wafer is1×10⁶ cm⁻² or less. In addition, the gallium nitride wafer has a size ofan area of 1 cm² or more. The full width at half maximum of an X-rayrocking curve (XRD) falls below 100 seconds.

The above gallium nitride wafer is used to fabricate an epitaxialsubstrate. The epitaxial substrate includes one or more gallium nitridesemiconductor films provided on the gallium nitride wafer. The galliumnitride wafer can be used as a substrate of, for example, opticaldevices (such as light-emitting diodes and laser diodes), electronicdevices (such as rectifiers, bipolar transistors, field-effecttransistors, and HEMTs), semiconductor sensors (such as temperaturesensors, pressure sensors, radiation sensors, and visible/ultravioletlight detectors), SAW devices, vibrators, resonators, oscillators, MEMScomponents, and piezoelectric actuators.

Example 7

In a growth furnace capable of locally heating a gallium nitridesubstrate, as shown in FIG. 5, a gallium nitride crystal was grown byHVPE. The gallium nitride substrate was prepared such that the growthsurface was not planar but had three-dimensional growth pits defined byfacets and composites thereof and such that the crystal was facet-grownwithout filling these pits and the composites, whereby dislocations wereconcentrated in the above pits and the composites. Accordingly, thegallium nitride substrate had dislocation-concentrated regions. Thegallium nitride substrate had a size of 2 inches and a thickness of 400μm, and its primary surface was the (0001) plane. The gallium nitridesubstrate, serving as a seed crystal substrate, was placed on a stage ofa support in a reaction tube. The substrate temperature was set to1,200° C. Gallium chloride was produced by heating a gallium boat filledwith gallium to 800° C. while supplying HCl gas and H₂ gas. A galliumnitride crystal was grown into a thickness of 400 μm on the seed crystalsubstrate by supplying the gallium chloride gas and ammonia gas. Thepartial pressures of HCl and NH₃ were adjusted such that the growth ratereached 50 μm/h or more without polycrystal formation. A crystal qualityanalysis of the resulting gallium nitride crystal by X-raydiffractometry (XRD) showed that the full width at half maximum of arocking curve for the (004) plane was 30 seconds, which is an excellentlevel. Then, the surface of the formed gallium nitride crystal wasetched with a KOH—NaOH mixed melt at 350° C. The dislocation density wasevaluated by counting the number of etch pits on the etched surface;they appear at positions corresponding to dislocations. The dislocationdensity was 1×10⁵ cm⁻², meaning that a low-dislocation-density galliumnitride crystal was grown.

Example 8

A gallium nitride crystal was grown under the same conditions as thoseused in Example 7 except that the crystal was grown into a thickness of10 mm. The resultant gallium nitride crystal was sliced in a directionparallel to the surface of the seed-crystal substrate and wasmirror-polished, thus preparing ten gallium nitride crystal substrateswith a thickness of 400 μm. None of the ten substrates was crackedduring the polishing; the polishing yield was 100%. One of the tensubstrates was picked out and etched with a KOH—NaOH mixed melt at 350°C. The dislocation density was evaluated by counting the number of etchpits on the etched surface; they appear at positions corresponding todislocations. The dislocation density was 1×10⁵ cm⁻², meaning that alow-dislocation-density gallium nitride crystal was grown. A crystalquality analysis of the gallium nitride crystal by X-ray diffractometry(XRD) showed that the full width at half maximum of a rocking curve forthe (004) plane was 30 seconds, which is an excellent level.

According to this embodiment, a gallium nitride crystal with lessdislocation density, high crystallinity, and resistance to crackingduring polishing after slicing is provided because the gallium nitridecrystal is grown at high temperature without forming recesses or a maskon the seed crystal substrate.

The embodiments according to the present invention are not limited tothe specific embodiments that have been described. In addition to theembodiments that have been described, an additional embodiment accordingto the present invention will be further described. Also, in theadditional embodiment, a gallium nitride substrate including first andsecond regions is used, and a primary surface of the gallium nitridesubstrate includes first and second areas. A seed crystal substrate isprepared by forming recesses in the first areas on the gallium nitridesubstrate and forming a mask over the recesses. Gallium nitride can alsobe grown on the seed crystal substrate. This method enables crystalgrowth under such growth conditions (source gas conditions) that thefirst areas are not inherited. For example, a crystal that does notinherit the first areas can be grown under such source gas conditionsthat the growth rate reaches 200 μm/h or more. This method has a costadvantage with increased throughput as compared to the method in whicheither recesses or a mask is used or the method in which no recesses ormask is formed.

In the embodiments that have been described, prolonged crystal growth ona seed crystal substrate is not easy at a substrate temperature higherthan 1,250° C. and equal to or lower than 1,300° C. due to damage to theseed crystal substrate, and it can also result in a decrease in growthrate due to an increase in decomposition rate. The method according tothe additional embodiment, in contrast, causes no decrease in growthrate in the above temperature range because a crystal that does notinherit the first areas can be grown even if the partial pressures ofthe source gases are increased in order to increase the formation rateagainst the increased decomposition rate.

Example 9

FIG. 12 includes diagrams illustrating main steps of preparing the seedcrystal substrate according to the additional embodiment. Referring toPart (A) of FIG. 12, a gallium nitride substrate 85 of the same type asused in Example 1 was prepared. The gallium nitride substrate 85included first regions 85 a and second regions 85 b that werealternately arranged. When the gallium nitride substrate 85 was etchedwith a melt containing potassium hydroxide, the inverted-polarityregions 85 a of the gallium nitride substrate 85 (the surfaces of theinverted regions 85 a correspond to the first areas) were selectivelyetched, whereby recesses (for example, grooves) 87 corresponding to theinverted-phase regions 85 a were formed, as shown in Part (B) of FIG.12. Referring then to Part (C) of FIG. 12, a mask film 89 is formed overthe entire primary surface of the gallium nitride substrate 85. The maskfilm 89 used can be made of an insulating material such as a SiO₂ film.The mask film 89 included first portions 89 a formed in the secondareas, corresponding to the surfaces of the uninverted-polarity regions85 b, second portions 89 b formed on side surfaces 87 a of the recesses87, and third portions 89 c formed on bottom surfaces 87 b of therecesses 87. After the mask film 89 was grown, the gallium nitridesubstrate 11 a and the mask film 89 a were polished to remove the maskfilm 89 a from the portions other than the recesses, as shown in Part(D) of FIG. 12. As a result, the first portions 89 a of the mask film 89disappeared, the second portions 89 b remained on the side surfaces 87 aof the recesses 87, and the third portions 89 c remained on the bottomsurfaces 87 b of the recesses 87. After the polishing, a damaged layeron the surface of the gallium nitride substrate was removed to prepare aseed crystal substrate 91. Referring to Part (E) of FIG. 12, the seedcrystal substrate 91 included recesses 93 corresponding to theinverted-phase regions and covered with the mask film 87 and polishedsecond areas 91 a.

The inverted-polarity regions 85 a can provide an N-plane on the primarysurface, whereas the uninverted-polarity regions 85 b can provide aGa-plane on the primary surface. Alternatively, the inverted-polarityregions 85 a may have a higher dislocation density than theuninverted-polarity regions 85 b.

Subsequently, a gallium nitride crystal was grown. The growth conditionsused were the same as those used in Example 1 except that the partialpressures of HCl and NH₃ were adjusted such that the growth rate reached200 μm/h or more without polycrystal formation. The surface of theresultant gallium nitride crystal was etched with a KOH—NaOH mixed meltat 350° C. The dislocation density was evaluated by counting the numberof etch pits on the etched surface; they appear at positionscorresponding to dislocations. The dislocation density was 5×10⁵ cm⁻²,meaning that a low-dislocation-density gallium nitride crystal wasgrown. A crystal quality analysis of the gallium nitride crystal byX-ray diffractometry (XRD) showed that the full width at half maximum ofa rocking curve for the (004) plane was 50 seconds, which is anexcellent level. When the gallium nitride crystal was immersed in a meltcontaining potassium hydroxide, the observation of the etched surfaceshowed that no recesses corresponding to inverted-polarity regions wereformed, meaning that a crystal containing no inverted phase was grown.

While the principle of the present invention has been described in thepreferred embodiments with reference to the drawings, it will beappreciated by those skilled in the art that the arrangements anddetails of the invention can be changed without departing from itsprinciple. The present invention is not limited to the specificstructures disclosed in the embodiments. Hence, all modifications andchanges resulting from the scope of the claims and their spirits areclaimed.

INDUSTRIAL APPLICABILITY

The present invention can provide a method for fabricating a galliumnitride crystal with low dislocation density, high crystallinity, andresistance to cracking during polishing after slicing by growing thegallium nitride crystal on a seed crystal substrate includingdislocation-concentrated regions or inverted regions.

1-78. (canceled)
 79. A single-crystal gallium nitride wafer manufacturedby a method for fabricating a gallium nitride crystal, comprising thesteps of: preparing a gallium nitride substrate including first regions,a second region, and a primary surface, the first regions comprisingsingle-crystal gallium nitride, the second region comprisingsingle-crystal gallium nitride, a crystal axis of the single-crystalgallium nitride being opposite in orientation to that of thesingle-crystal gallium nitride in the first regions, the first regionsbeing exposed in the first areas, and the second region being exposed inthe second area; forming recesses in the first areas and a mask over therecesses in the first areas to prepare a seed crystal substrate; growinga gallium nitride crystal on the seed crystal substrate by liquid-phasedeposition or vapor-phase deposition such that voids corresponding tothe recesses are formed; separating the gallium nitride crystal from anintegrated piece of the gallium nitride crystal and the seed crystalsubstrate; and forming a single-crystal gallium nitride wafer from theseparated gallium nitride crystal, wherein the crystal axis of thesingle-crystal gallium nitride has the same orientation in any region ofthe gallium nitride wafer.
 80. A single-crystal gallium nitride wafermanufactured by a method for fabricating a gallium nitride crystal,comprising the steps of: preparing a gallium nitride substrate includingfirst regions, a second region, and a primary surface, the first regionscomprising single-crystal gallium nitride, the second region comprisingsingle-crystal gallium nitride, a crystal axis of the single-crystalgallium nitride being opposite in orientation to that of thesingle-crystal gallium nitride in the first regions, the first regionsbeing exposed in the first areas, and the second region being exposed inthe second area; forming recesses in the first areas and a mask over therecesses in the first areas to prepare a seed crystal substrate; growinga gallium nitride crystal on the seed crystal substrate by vapor-phasedeposition at a growth temperature higher than 1,100° C. and equal to orlower than 1,300° C.; separating the gallium nitride crystal from anintegrated piece of the gallium nitride crystal and the seed crystalsubstrate; and forming a single-crystal gallium nitride wafer from theseparated gallium nitride crystal, wherein the crystal axis of thesingle-crystal gallium nitride has the same orientation in any region ofthe gallium nitride wafer.
 81. The gallium nitride wafer according toclaim 79, wherein the size of the gallium nitride wafer is 1 cm² ormore.
 82. The gallium nitride wafer according to claim 80, wherein thesize of the gallium nitride wafer is 1 cm² or more.